This post is long overdue, but since the question keeps popping up on various SharePoint discussion lists, I figured I'd document the answer here (next time I can just post a link instead of typing the whole thing out again).
In a SPD workflow, you cannot trigger a SharePoint workflow when a column changes; you can only use the ItemChanged event. To get more granular, then, you need to add some extra bits.
Let's say you have a list called "5K Races" with a column called StartTime, and you want to execute some actions when the StartTime value changes. Simply perform the following steps:
1) Create an additional column (same datatype) called OldStartTime.
2) When the workflow starts, compare StartTime to OldStartTime.
a) If they are equal, then do nothing (end).
b) If they are NOT equal, proceed with your workflow.
3) If 2b, then set OldStartTime to the value of StartTime.
By performing step 3, you ensure that by the end of the workflow, OldStartTime will be equal to StartTime -- this is important because the workflow will continue to run every time a particular item is changed, but by taking away the criterion that would cause the workflow to run the second time, you have avoided an endless loop situation.
Monday morning, the calls started. For some reason, long-time users were unable to edit list items. I figured we had a permissions issue, so I popped in to look at the Site Settings -- and found that I couldn't. A quick trip to Central Administration showed that I was still listed as a Site Collection Administrator, but I had no power at all on the site collection in question.
A quick glance at the logs told me that the server had recently shut down unexpectedly (this is a Hyper-V virtual machine). Apparently, in the confusion, somehow SharePoint decided to lock the site collection as Read Only. This can be remedied in one of two ways:
1) In Central Administration, go to Application Management->SharePoint Site Management->Site collection quotas and locks. Once you have arrived, select the correct application and site collection, and you will have the opportunity to view and set the lock status of the collection (it most likely will be set to "Read-only", and you'll want to move that radio button to "Not locked").
2) Fire up stsadm and issue the following command:
stsadm -o setsitelock -url http://myportalsitecollection -lock none