I am running three different blogs, and two of them have this problem. The problem occurs when writing the body to a new page or post. The active tab will be set to "Visual" but I have no formatting options, and I can only write as if I'm in the HTML tab. HTML tags work fine.
EDIT: Here's a screenshot: http://img152.imageshack.us/my.php?image=screenea7.png
No one else is having this issue, or have heard about it?
Maybe there's an updated version of the TinyMCE editor or something that I'm missing?
I don't know whether it's any help but I had a sort of similar problem for a while.
It turned out to be because some of my page titles were really long. And so when I went into a page's edit screen, those titles were being output as options in one of the drop-downs on the right. Even though they weren't visible, their length was forcing the layout to shift around, resulting in my HTML button disappearing. So have a good look at all the fields in your edit page to make sure there's nothing like that creating stylesheet issues.
Other than that, I guess maybe you have a dodgy plugin that includes a a TinyMCE button?
The fact it's happening on two of your blogs: what's the common denominator?