Originally Posted by GUWonder

United will match AA Gold with United Premier generally.
USAir will match AA Gold with US Air Silver
Continental will match AA Gold with Continental Silver.
Delta used to status match AA Gold with DL Silver but now seems to be doing challenges mostly where you have to do a certain amount of DL flying to get status.

An airline will generally upgrade your account only once -- once in an account lifetime -- with a status-match from another airline. The status match is good through at least the end of February of the following year. Some status matches will result in status that is good for more than one membership year, but that usually involves asking for a status match in the the latter part of the year.