yeah, everyone 'just does', sort of like how the entier galixy is populated with humans, without any realy good reason for it. that was one of the coolest things about SG1 origonaly, there were humans all over the galixy, not aliens that just happened to look like us and have the exact biological and biochemical makeup, and there was a good reason for it, and _nobody_ spoke english, well almost nobody. now stargate is just stupid generic scifi... sad 
Well, obviously, except it takes more than just 'generic sci-fi' to become the longest running American Sci-Fi show ever, and globally second only to Doctor Who. Honestly, i've seen so many people raise this question and say it really lowers the quality of the show, but you'd be saying the exact same thing if
nobody spoke English. I mean, having to translate everything in every off-world mission would get real tedious and real old real ****ing fast.
The English thing aside, Pegasus is populated not by beings that *look* like humans, but they
are humans. The main idea is that the Ancients seeded the barren Pegasus Galaxy with Humans, the more primitive form of the Ancients themselves. They did the same thing with the Milky Way, only to a smaller extent.
@Nuke: AFAIK, this episode won't be shown until mid January when Atlantis starts airing again on SFC. However, it's been airing up in Canada for a few weeks now, and thus open to acquisition... *ahem* elsewhere.