Once in a while, when I hit command-I in DEVONthink Pro, it just beeps. Nope, can’t do italics. And when I look in the Format > Style menu, the italics option is greyed out!!! What does this mean? Why is this happening? And it applies to ALL documents. (Yes, they’re all RTF.)
Thanks in advance!
You might have a look at this thread:
http://www.devon-technologies.com/scripts/userforum/viewtopic.php?f=3&t=6786&p=30950
Yes, just like that user, I have to quit and restart.
Every time it occurs. 
Has been anything logged to the system console? Which font are you using?
Ordinary Times, in this case. Optima, in other cases. These fonts allow me to italicize SOMETIMES, but other times, italics is greyed out, and I have to quit-and-restart. 
Strange. Are other styles like bold/underlined/strike available?
Yes. Whenever this greying-out happens, all the other styles are both available and functional. It is indeed strange. 
I’m running out of ideas, especially as all those commands are (more or less) completely handled by Mac OS X. Maybe a font cache is damaged or there are duplicate/damaged fonts installed on your s ystem?