Thanks for the reply Lisa - very helpful. BTW, I had just purchased an HTC Sensation a couple of days before my post on 7/17 (in large part due to your review). I just checked with T-Mobile about their Wi-Fi calling. They have a kind of strange procedure - where when you turn on Wi-Fi it gives a message saying that it uses minutes from your plan. So I called them to understand why they would do this. The person on the other end - indicated that it doesn't use plans from your minutes if you are "properly" connected to the Internet. They have no FAQ or document on what "proplery' means. They indicated that as of now the only way to tell is to make your Wi-Fi calls and then check your minutes using #646# to see if usage increased.
I do wish that you could use your Sensation as a Hot-Spot and use that Hot-Spot to make calls from your Sensation (I guess it kind-of makes sense that you can't, otherwise the phone would have to recognize it's own W-Fi signal. However, I would imagine this should be possible if engineeered properly, i.e. have separate Wi-Fi and phone radios)
As for Hot-Spots - after much thought and research - I decided to go with T-Mobile's. The primary reason is coverage. I hate that the carriers are ripping us off for data with these new Tiered plans. So if I have to pay - I'd rather pay for something that has more coverage and fairly high reliability - right now that is T-Mobile. I would go with Sprint's unlimited plan but for now their coverage of the country is not as extensive as T-Mobile's and their Wi-Max has too many issues with coverage inside of buildings, etc.