I've been using my iPhone for about a week now and I've noticed quite a few bugs. Although I wouldn't really call most of them bugs. My opinion is that since you never really close a program, you just "minimize" it, for lack of a better term, eventually you bog the system down.
I've read anecdotes of people complaining that they were watching Lost and talking on the phone and had 5 web pages open, and couldn't figure out why it became unresponsive.
I had this slider problem too, and my best advice is to power down the iPhone every couple of days and restart it, much like you'd do with your home computer. From what I've read, and from personal experience, this seems to solve 99% of the problems. It would also help to remember that while this is an amazing phone/ipod, it is not a full fledged computer and thus, does have limitations. Remembering that will stave off loads of frustration.
S.