I could see this. It does seem to get progressively worse as time goes by. Mine certainly did.
The first time it happened for me was on a very hot day, with the sun beating down from pretty much directly overhead. For the next month or so it seemed to do this each time I went riding in direct sunlight around the middle of the day. By the time I got it replaced, it didn't seem to matter when I rode the bike so long as it was warm out. Cold days it had less issue.
Had the sticking speedo problem for the first time a little over a week ago (reading 90 while going maybe 50; reading 40 while at a complete stop), then had it happen again this past weekend. Both times it reset and began to operate properly after shutting the engine off and restarting. I have about 3700 miles on the bike since I bought it in December 2010.
Contacted the dealer and sent a pic of the speedo showing nearly 40 mph while at a complete stop. They just heard back from Triumph and it sounds like the company is 3-4 weeks away from finalizing a "fix" for the problem. Reading about the problems many of you have had, I wasn't convinced that simply changing out the speedometer was the best fix. That procedure has worked for some, but not for others.
Sounds like Triumph doesn't think that the speedo changeout is the ultimate fix either (although that could still be necessary). He said they have narrowed it down to specific components (unfortunately, this was over the phone, and despite being an engineer in my past life, I'm really bad at details if I don't have an email in hand at this point). They told the service manager to wait for 2-3 weeks, file the claim, and by then the fix should be worked out. Also, they estimated that maybe 1 in 20 bikes have had the problem.
By the way, both times it's happened to me, I had been on longer rides (more than 30-40 miles) in direct sunlight in the middle of the day. If I hear anything, I'll update this.