Using DTPO latest beta on 10.5.7. I can bold in rtf files by using the right click, but not using the shortcut key. It is also grayed out in the menu.
Putting together this issue with your post about not being able to clear the ‘Open Recent’ menu, I suspect a problem in the software environment on your computer.
See if those problems go away when you switch to the Guest user account. If so, you might try to identify some software component that’s causing OS X errors – perhaps a plugin and/or third-party preference pane that should be removed, or that may need an update after the latest OS X update…
I’m having the same problem also, intermittently.
When I get a chance, I’ll experiment as a Guest User. But wanted to chime in and mention that Philosopher_Dog is not the only “unemboldened” one, here. 
If you can identify the software that’s causing the problem, please report it here.
Ok. I’ll try to log in as guest and report back what I find.
Actually bolding seems to be working now. I did recently remove PTHPasteBoard. Perhaps that was causing some problems?
Open recents not clearning is probably a separate problem.
I’ve given up trying to clearn separate problems. 