Verizon doesn't use SIM cards - it's CDMA like Sprint.

Sprint is not the largest cellular network. Sprint is either third or fourth largest in the US in terms of customers and covers both less people and less geographic area than AT&T or Verizon. Plus, Sprint isn't technically a cellular carrier. To split hairs, cellular service is at 800MHz while PCS service is at 1900MHz (which is what Sprint uses for their CDMA network - their iDEN network being a SMR network). Normally, I wouldn't nitpick like that, but Sprint made a big deal about being all-PCS a while back calling themselves "the clear alternative to cellular".

HTC has had good relations with Sprint so I'm guessing that a Sprint version might come, but it won't come quickly. Sprint is losing customers at a high rate. There's no incentive for a phone manufacturer to want to put effort into debuting on Sprint given that the line of phones (Pre, Hero, Moment - which were all great phones) Sprint has gotten haven't done so well.

T-Mobile is easy to start out with because they don't require lots of customization and GSM phones can be re-used all over the world. Verizon customers are clamoring for an iPhone competitor (as evidenced by Droid sales). AT&T has the iPhone. Sprint just isn't winning customers. So, it's to be expected that Sprint's going to be at the bottom of the heap after lackluster results for quite a while.

Everyone thinks that they are interchangeable names, but they are different. UMTS is not coming to the United States presumably ever. First - What is it? UMTS is the name given to WCDMA when it is in the 2.1GHz spectrum band. That band is occupied in the US with no sign that it will ever be freed up. The US will get WCDMA, bit it will be like our version of GSM: on another frequency that means that we will need to get world phones. God forbid the FCC should open up the frequency that the rest of the world has agreed upon (or at least all of Europe has agreed upon).