How can I ensure reliable updates for an Android phone/device? My Android phone has been abandoned by the manufacturer and carrier. It's stuck on 2.2, and doesn't seem likely to get any updates in the future, nor can I find updated ROMs for it.

I'd be willing to purchase a new phone, but how can I ensure that it will be supported? Does Google, or any carrier, or any manufacturer, make any guarantees about providing timely updates for their phones?
The only devices that you can be confident about getting timely updates for are Nexus devices. These tend to get updates for at least 18 months and they receive them as soon as Google releases them (they're essentially Google devices made by other manufacturers).

All other manufacturers take a while to release an update once it is released by Google, typical 3+ months. They also have less incentive to release updates as they want you to buy their latest-and-greatest new phone.