Yup. Happened to my once a few days ago on my orig iphone with 2.0. I think the 2.0 firmware has a bad memory leak. I'm to the point of having to reset or turn off(red slider)/on every day or so.
I've also noticed that some sounds don't play after a while. For example, all the click sounds when you turn the iphone on and off go silent after a while, but are restored after I reset or turn off/on.
2.0.1 should be here soon, I think.