Teemu brought up the current event declaration behavior today, and even pointed to a bug report about it.
I'm going to take the opposite stance. putting the onclick="whatever" attribute on the tag is the best solution to a complicated problem. Now, if the problem were limited to the points that Teemu addresses, then yes, it looks like it should changable based on the current dev model. However, the thing that makes this problem more complicated is that not every control is at the top level of your page. Asp.net uses a hierarchal declaration system with containers and child controls. One by-product of this system is that not all design-time-selectable controls on your page have references at the page level, or even refer to a single instance of a control ( in the case of a databound template declaration ).
So, if you drag a button onto your form, and double click it to add a hook to the Click event, there are 2 or 3 basic choices ( depending on language ( VB has the Handles thing going on ))... add the glue to the tag, or add it in some initialization code block. Some want to use the code block because they don't like their UI declration to know the name of the handler function of the event.
However, the hole there is that if you now move that button from the top level of the page's heirarchy, into a INamingContainer like a Repeater's ItemTemplate, then the instance referenced in the initialization-block doesn't exist. ( VB's Handles doodad breaks here too )
So how should the designer attach to events in child controls if there is no top level instance to code against, nor any guarentee that the child control will exist at any given initialization time ( or be instantiated at all during that request for that matter )? To me, it seems it can't ensure correctness with any way but declarative. Which follows to the next point... Should it really use two completely different hookup mechanisms for top-level and child controls? I assert that it shouldn't as that would lead to some nitemare maintainability problems. ( "I know that it's hooked up somewhere, where did that code block go now?" ) Therefore... the tag declaration is the correct place for the designer to hook up event handlers.