How can you say that Terrans are immobile? They can go/build anywhere! You can pop a small installation just about anywhere, and it's more effective than a hastily-tossed out Protoss base, and easier to place than a massive space-using Zerg base.
Zerg's highest mobility trait is how amazingly fast you can transport troops with Nydus canals...but I'd think Terran is faster...
I'll admit that having your base HQ building being only 300 instead of 400 is definitely helpful, but it has some drawbacks too. As far as unit movement, I will admit that Zerg are fastest, with their 3 chief ground units all having speed upgrades, but Protoss are almost as good. Scouts are ridiculously fast with their speed upgrade, and I believe upgraded Shuttles are almost as fast, certainly the fastest transport unit. Shoot, Protoss even has the fastest "flagship" type, as the Carrier is significantly faster than the Battlecruiser and waay faster than the Guardian (which is the clostest thing to a flagship that the Zerg have).
But then, we were talking about how mobile bases are. Really, Terran is the ONLY mobile base race in SC1. Zerg buildings can SPREAD, but they can't move. Protoss can warp in a lot of stuff really quickly, but they don't MOVE. Terrans are the only ones who move in SC1, which is obviously changing in SC2...and that's all I was really trying to get at in the FIRST place.