I have same problem, left you note on your website just now and then found this discussion (sorry for duplicating). This is when I go into EDIT or ADD NEW POST.
I thought at first it was conflict with TINYMCE, but it does it with just RC as only plugin running (I deactivated the other 45).
In my case, I'm running in Firefox 8 and IE8. Appeared to be same problem in both, but then IE8 "switched over" to visual mode after about 15 seconds. FF didn't switch, but when I reloaded the page, this time it worked.
FWIW, I'm running Weaver as my theme (the full weaver, not simply 2010).
If you have any thoughts, let me know...I'm thinking it's some sort of conflict with FF though -- I only recently upgraded my FF (I run it on work computer that has painful restrictions normally) and I don't think it happened before that.