References to Bulgaria aren't that uncommon. I remember that in MacGyver (my favorite series after SG-1) it was referenced once, (they were probably just using the name of a country no one's heard of) and later on an entire episode was shot here. And the action took place here as well - I think so they could show a communist country without going to Russia. That was about 20 years ago. Also, filming here is cheap. In that line of thought I think one or more episodes of Eureka were shot here, though it's not referenced in the show. Part of the Hitman movie too. Even I played a Gaul warrior once.
But back to references. I think Bulgaria and Bulgarian are used mostly as something unknown or something people have only heard of. A far away place, less familiar than Russia or the middle east. And that's the "funny" part, with most responses being "Where's that?" or even "You're just making it up", because the unfamiliar always has comic potential. I also think that now that you know about Bulgaria, you're just noticing it more often.
I don't I've watched that Psych episode. And I'd probably watch Valentine's Day just to see if it's really Bulgarian they speak. I remember a similar joke in a Bulgarian series called Морска сол (Sea salt): An armed and not very bright guy bursts into a room, waves his gun at the four people there and shouts:
- (in English) Hey! Hands up! *mumbles indistinct English sounds* OK?
- Какво? (What?)
- Уча английски. (I'm studying English.)

Also, check this trailer out: http://www.youtube.com/watch?v=iYE3HQGMWSA It's the first Bulgarian (well, Bulgarian-English) movie I'm actually hopeful about. Not that we don't have good ones, but I never saw trailers for them.