just to add evidence to this thread, I had the exact same problem twice.
The first time, I did a hard reset (after two weeks of use). The second time, when I was about to smash this damn phone against the wall, I found this thread and deleted a couple apps I was not using until I had less than 70. I rebooted the phone and BINGO! it is working again. I think I had around 75 apps when it started to give me problems.
I rarely reboot my phone, but when I did it, it was gone. It would force close almost everything. After I uninstall a couple of my apps it is back in business.
I don't know if the problem is Android or Sense, but this limitation is annoying. I don't think that 70 apps is a lot. Also, the 150mb of internal memory is a joke. If I have 16gb of memory in my SD why can't I save my apps and emails to it?? I know, there is a lot of technical explanations behind it...but that is lame anyway.
I was so happy with Android but now that I have used it for a while I am finding lots of limitations. That is sad... hope Android 2.0 is better....