Okay, this is a bit embarrassing. After posting both here on my blog and writing to the SIMILE listserv, it now appears that the Google Form Timeline is not functioning as it should. There have been at least three people who have used the form to add an event to the spreadsheet, but they are not being plotted on the timeline.
At first I thought that it had to do with the way the dates were formatted: the spreadsheet was forcing them into a format that Exhibit doesn't read. However, I've corrected that and I'm getting nothing, not even events that I've added subsequently added directly to the spreadsheet or via the form.
I tested this process with three different people this afternoon and didn't have any problems. Firebug isn't showing me anything wrong with the page. I'll work on hunting down what the problem might be tomorrow afternoon.
Thanks for the interest thus far.