Android :: Google / Carriers / Manufacturer Relationships / Support

I'm about to dump ATT/IPhone for Sprint/Android (EVO), but one thing has been nagging me and I want to make sure I know my options before jumping (e.g. given these 2 yr contracts).My concern is with ongoing support for my phone after purchase, support in terms of receiving the latest/greatest updates to the Android platform (e.g. within hardware constraints). Who ultimately controls whether a phone gets a release (e.g. Carrier or Manufacturer)?It seems to me the best bet is to go with the Nexus One platform given that is under the control of Google and should be well supported in that respect. However, it seems like it is going to be a hassle and/or costly to go with Nexus One using Sprint. Is there anyway around paying full price for Nexus One and using Sprint as a carrier? Any chance Google has a deal with the carriers to subsidize phones?Plus, I would prefer not to have the blot (e.g. UI modifications, app replacements, etc) added to the phone by Carriers and/or Manufactures (e.g. prefer Android as perceived by Google). Wondering who has the control of UI modifications. Does the Carrier contract with the Manufacturer and have control over what is added and how it is supported (e.g. whether a phone gets a new Android release, etc)? Or does the Manufacturer have the control?And lastly, say I get the EVO and on down the line someone (Manufacturer or Carrier) decides no more updates for the EVO. It sounds like I should be able to root the phone and continue with updates. Without support from Carrier/Manufacturer of course, but is it against the Carriers rules to root a phone?
Also, when rooting a phone, I'm assuming you can not take a release straight from Google and install it. Rather, someone has to implement the release for the target hardware (e.g. build, add any necessary drivers, etc). Are there any gotchas in doing this? Who is the most reliable supplier of these builds? I know that is a pretty broad question and I'll plan on doing additional research in that areas, but if anybody can give me a nudge or share their experience, that would be great.