For some reason I (and a few other users... Galland and Thantsants off the top of my head) experience problems when composing long posts.
After a certain length, the text field automatically scrolls up one line so that the cursor is always out of sight.
I don't have the problem on OSX with Safari, only on Windows 7 and Internet Explorer 8. Shifting to Firefox helps, the problem does not manifest.
Edit: Oh, yeah - the post length needs to be twice the amount of rows of the edit box.