For what it's worth, it's happening to me even on tabs **that are active**.
For example, when I began typing this very reply, I had literally typed "For wh" and then suddenly I was back at the Apple Discussions "Log In" page! The tab / page into which I was typing was reloaded right in front of me!
I'm running: Safari 5.1 (6534.50), 10.6.8 (10K549), MacBookPro8,2. I have NO extensions. None. Java is off. JavaScript is on (because try doing anything these days without JavaScript).
It's happened to me just like this on many other web apps... BMC Remedy my best example. I've had to re-create a huge amount of data because I've had to re-enter it when the page refreshed while I was actively typing into a form field.
This did NOT happen under the version of Safari previous to this. (I forget what version that was... 5.0?) I started noticing it as soon as I upgraded to Safari 5.1.
Also - I'm a bit of an unusual case. Right now I have 21 open windows with a total of 162 tabs. Again, this was NOT a problem under the previous version of Safari.