I was going to hold off on posting this until the public beta drops, but anyone preparing for the drop will want to get the right hardware, OS and optionally virtualization in place now. Then when the beta drops I'll write more about specific steps to get SQL, SharePoint and your development tools installed.
Hardware and Operating System
The "official" hardware or virtualization requirements for running either SharePoint Server 2010 or WSS 4.0 are: a 64-bit dual-core 3 GHz CPU, 4 GB of RAM, an 80 GB hard drive, and a DVD drive (presumably for installation). My first virtual environment ran fine with 2 GB RAM and a 20 GB (virtual) hard drive, though as always more is better. Remember that like any product, SharePoint and Visual Studio 2010 are not fully optimized during the betas, and performance will continue to improve through RTM.
You do need a 64-bit OS and for virtualized hosting therefore have the choice between Microsoft Hyper-V, VMWare Workstation, and Sun’s VirtualBox (and likely other third-party apps, though I know these work). Out of the game and being deprecated in deference to Hyper-V are Virtual PC and Virtual Server, neither of which support a 64-bit guest OS. There are a few options for operating system with Windows 2008 R2 being the clear favourite, though Windows 7 and Vista – for the first time non-server operating systems – are supported for WSS 4.0 and SharePoint Server development.
Minimum SQL flavours are SQL Server 2005 with SP2 or SQL Server 2008 Standard Edition, though standalone installations can run on SQL Server 2008 Express (included). Note that some features (like converting a Site Collection to read-only mode) require SQL Server 2008 Standard. While for performance reasons it is not generally recommended to run SQL Server (or the SharePoint Index role) virtualized, it's normal in a development environment.
Additional requirements included in (or downloaded by) the installer are: .NET 3.5 SP1, Silverlight 2, IDFX “Geneva”, Powershell 1.0, and IIS 7.0. Supported client browsers are IE 7, Firefox 3 and Safari 3.
Windows 2008 R2 with Hyper-V is a good choice for virtualization on desktop or server installs, but is not supported by Microsoft for use on laptops. Hyper-V was really never intended for use on laptops, and so conveniences like Sleep and Hibernation aren’t there. Other notable things not usable on Windows 2008 R2 are Bluetooth, Zune software and Call of Duty, and there are others. I really like Windows 7 but I don't want to do development on my daily-use OS, so third-party virtualization like VirtualBox or VMWare are the clear winners for me, both of which support a 64-bit client OS. That said, many of the SharePoint MVPs who presented at SharePoint Conference 2009 (#spc09) use a configuration that can dual-boot into either Windows Server for development and presentations, or into Windows 7 for personal use. The fact that the boot times are great for both minimizes their dependency on Sleep.
Average hard disk sizes are larger, so virtualization solutions that support differencing disks - Hyper-V and VMWare - have a clear advantage. A differencing disk lets you create a single base image (a virtual hard disk) that you can then branch with many additional drive images - for example one for every project you work on. So your base image with SharePoint Server, SQL Server, Visual Studio 2010, and Office might be 20 to 30 GB, but additional machines will only be as large as your project and content, perhaps 5 to 10 GB apiece.
And for your physical hard disk do yourself a favour and get an Intel SSD (solid-state) drive installed. The average sizes are smaller (~120 GB is large as of writing this), but performance is absolutely killer with reports of up to +40% in boot times and regular operations; these drives are the reason people don't mind using Hyper-V to boot into their dev images. It's also nice that my laptop doesn't sound like a small aircraft anymore. All concerns about virtualizing on SSD have been wiped away (unlike when they first appeared on the market), so if you can get one, do it. As always any reliable 7200 RPM drive will perform well, but SSD is definitely the new "best-of" around. For external storage, my "building a machine" post for 2007 still applies with 7200 RPM eSATA through an ExpressCard interface probably your best bang-for-the-buck.
This section describes a few known configurations used by early adopters, feel free to comment with your own!
Hewlett-Packard HP8530. Quad-core with up to 8GB RAM and 7200 RPM disk(s), running Windows Server 2008 R2 and Hyper-V. Recommend dedicating 2 cores and 6 Gb to the guest OS (either Windows Server 2008 R2 or Windows 7).
IBM Lenovo T500 and W500. Intel Core 2 Duo T9600 / 2.8 GHz Dual Core, up to 8 GB RAM, 15.4" screen, SSD optional, ~6.2 lbs. These are nearly the same laptop, the W500 has a better screen and video sub-system (find a detailed comparison here). The differences weren't important to me so I'm using the T500 and it's been great. My current virtual machine is assigned a 20 GB hard drive, 4 GB RAM and one processor (I tried two but since I only have two it was unstable, at least with VirtualBox; VMWare generally does better sharing CPU cycles with the host).
Servers and Desktop Workstations. With relative costs, remoting into a dedicated machine or a virtual server hosting many guests is sounding better and better these days, especially for development and test farms. The principles are the same, the requirements are the same, but it no longer matters how or from where you connect, and hardware costs are significantly lower. For anyone who doesn't need to demo or present on a laptop, I'd seriously consider it.
Until the Public Beta
With any luck that should get you started, have fun!