All the UMTS 3g phones I have used have being quite happy with a reasonably recent 2G GSM sim card in them, and they all seem to support GSM just fine.
The problem I have with phones is that lots of them do not have a UMTS or HSPA only option in the menus, just an option to turn it off and go back to GSM only, which is hopeless IMO.
Getting a phone with the manufacturers software on it means you miss out on all the network specific stuff, like the custom internet things, streaming stuff, music store stuff etc. I find that a total non issue so dont care, but some people love that stuff.