(It took me about a months to finally figure this one out.)
I have a separate page in my blog that is basically an ordered list of links. The list has grown quite long (over 550 lines).
Very recently, when I went to update my list, I discovered that the formatting toolbar (b, i, link...) above the textbox had vanished. Disabling all my plugins (about 15) brought it back, but no particular plugin was the one culprit.
Only by loading other shorter pages did I discover the toolbar was fine and appearing normally.
Apparently, when a page gets "too long", some obscure buffer overload is preventing the Formatting Toolbar from loading.