I tend to joke around and do silly voices and impressions all the time, but especially when I'm gaming.
As a player, I've been complemented on my ability to keep up an accent while remaining conversational (and vocal) throughout sessions. As a DM, my abilities get stretched a little thin, however, and I become somewhat self concious about trying out new voices for fear they sound like crap.
The two voices that got the most use belonged (not coincidentally) to the two characters I played in 2-year campaigns.
Anaphris, my Loresong Faen Magister (Roughly Gnome Wizard in D&D terms, if you're not familiar with Arcana Evolved) was my attempted impersonation of Malcolm McDowell, sort of a gravelly, cultured, menace to it, while kicking it up higher so that he would sound small. Many have noted that it sounds something like the lead singer of Prodigy, which resulted in "Firestarter" becoming his themesong. "The sreaming is just a reflex. Repeated tests have proven that the spell inflicts death far faster than the nerve conduction velocity"
Mayhew Engels, my Pseudo-Slavic Socialist Holy Liberator, had a rather thick Russian accent. While not perfect by any means, one of the other Players was a language major focusing on German and Russian, and she complimented it. Still, I had to make the occassional joke.
"Everything in life have purpose. Ski-vi-gi-ly (Squiggly) things purpose is to die, and make Mayhew look 'eroic!"
My biggest problem with voices is keeping Scottish and Irish straight (Thanks Mr. Doohan) and coming up with effective voices for Authority figures that don't seem utterly alike. Still working on that one.