IMHO,our military is stretched too thin right now to do the kind of job that would be needed in Iran. Then of course you have all of the domestic appeasers, etc. who would go all out to stop us from having another front in the war on terrorism. Lastly, even though regime change could work, it needs to come internally - even anti-regime forces are probably nationalistic and might not necessarily welcome our direct military involvement.
We may not have enough troop strength for an invasion, but we can shut down their nuclear weapons program for several years with powerful airstrikes. Then it's up to the next administration and Israel to make sure their nuke weapons program remains permanently shut down.