OK to clear this up a bit more, SMS has nothing to do with WIFI, it is completely network dependent. When you send a text while on UMA its still going through T-Mobile's cell network. Example: the Bold 9000 has WIFI but not UMA, if you do not have a network connection but you have a WIFI connection, you can NOT send or recieve any SMS. A UMA connection turns your router into a T-Mobile router so that you have great signal. On the other hand if you are using DATA services, UMA makes it so that web browsing is super fast, downloads quick, AND if you are browsing using the Hotspot browser then you are not actually going the BB server but rather directly to the page. Also, if you don't have a DATA plan, being on WIFI gives you all the abilities of having a DATA plan, IE: BBM, IM, Web surfing, downloading, and emails.
The advantage to having UMA on T-mobile phones is that 1) you get perfect reception when you are on UMA and at least for me, gives you signal where you might not have it(I live in a cellphone dead zone), and 2) if you pay for the Hotspot service, all the calls that origionate on UMA are completely FREE and don't count against your minutes.
If you are traveling abroad like China or something and you are on UMA, T-Mobile sees your phone as if you were still in the USA and therefor all your calls to or from the USA are billed as they would be normally which can save you hundreds in roaming fees.
Sorry for the rant, I used to work for T-Mobile :-)
100% agree on every point (and I have used the UMA to call the USA from China for free)