The way carriers control the update is because they dont approve it. Microsoft is the one with the infrastructure in place via Zune to physically push it but has given control to the carriers by allowing them to approve updates which was obviously a compromise they had to make to get WP on their networks. Anyone who thinks their carrier is the one actually releasing an update to their phone is mistaken.
When it comes to OEM devices, there are exceptions but it seems they are more likely to release all updates. My DVP which is labelled as Generic-US received 7740 and 8107 promptly. My other DVP which is labelled T-Mo US is still on 7720.
Solution? Buy/import an unlocked, unbranded, free of carrier contamination device. Sadly in the US the problem is two fold - there are hardly any options for the GSM carriers at&t and Tmo, and the average consumer wont pay more than $200, whereas unlocked phones are normally substantially more expensive. So the market in the US is virtually non-existent and the carriers know this and do everything they can to block it.
In Europe, you are able to buy carrier-free, with contract, or even carrier-free + contract at stores like phones4u, carphonewarehouse, expansys, etc giving you the choice.
Two hopes remain, though its only a hope, and that is when WP8 hits Nokia will be able to produce pentaband phones that can be imported to the US, like they have been doing on Symbian for years before they moved to WP. For people like me, that is the ideal solution but sadly for the vast majority that isnt feasible.
Secondly, Tmobile has recently stated that they will be refarming their network to offer HSPA+ on the 1900 band which opens up the market for unlocked at&t phones, but there you will still be subject to at&t's update schecule as it will still be a carrier phone, though SIM unlocked.
So basically, if you live in the US you are screwed if you want regular updates for WP. Its not as hackable as android or Windows Mobile to load custom roms so really you are left at the carrier's whim. And in this respect, Microoft has definitely not fulfilled the promise implied about Windows Phone updates being available to all but realistically there doesnt seem to be anything we can do about it.