There's also the new EPA efficiency requirements to deal with. Starting in January 2006, all central air and heating units must be SEER 13 or higher (currently, the minimum allowed is SEER 10). Even though the industry trade group spokesperson is saying this will only add a few percent to the cost of a unit (I think I read $171), around here installers are saying it's going to almost double the cost. For example, a replacement compressor that costs $1300 today will cost $2100 starting January (this was an actual quote given to a coworker today, BTW).
I'd like to replace my 10-year old gas furnace with a heat pump (or maybe one of the new hybrids that use gas instead of electric coils when in 'emergency' mode) but I figure the installation would cost $7000 - $10000. It would take decades to recoup those costs through the incremental annual savings.