DC has three airports, if you toss BWI into the mix, which we do because it's a hub for Southwest and AirTran flights. A bus-train combo will get you from Baltimore-Washington International to Union Station in DC in under an hour. Your next option is Ronald Reagan Washington National, which has lots of US Airways flights and has the benefit of being close to downtown. (DCA has connections to two Metro lines, in addition to the usual cabs and shuttles.)
Dulles is the third airport, with abundant JetBlue and United connections. (JetBlue is actually adding flights into Dulles specifically for the inauguration.) This is also where international flights are most likely to touch down. Word of warning: It's a terrible hassle to actually get to and from IAD.
Amtrak is actually a great idea in this case. The Northeast Corridor has excellent frequencies and reliability, at least compared to train service in the rest of the country. You can ride right to Union Station, just a few blocks from The Mall, or connect to the rest of DC there by Metro.
Because Amtrak along the East Coast is so reliable, some clever folks are actually planning to avoid the DC inauguration hotel scene and stay in Baltimore or even Philly, riding into town for day trips. Hey, if that's your thing, go for it.
Undoubtedly, some people will drive to DC. If you enjoy sharing the road with a guy shaving while doing 75, paying exorbitant parking fees and enduring horrific traffic, maybe you should too. Or, you know, don't bother--and rely on the city's excellent public transportation options once you get to town.
This last option is really only for the East Coasters, but it's definitely a viable one, as long as you don't try to travel on January 20. (We expect the entire city will be gridlocked that Tuesday.) Super cheap-yet-pleasant coaches operated by BoltBus, Megabus, DC2NY and, yes, even APEX will get you to DC for next to nothing, leaving your budget intact--so you can spend all your bucks on some floor space found on Craigslist.
· Joint Congressional Committee on Inaugural Ceremonies [Official Site]
· Comparing the Cheap Buses [IT]
· Inauguration 2009 [Expedia]
[Photo: Joint Congressional Committee on Inaugural Ceremonies]