The main things to worry about are gas, battery, and tires.
Yes, gas does go bad. Put Sta-Bil in the gas tank according to the directions on the bottle and then drive it for a mile or so then top off the gas tank. The Sta-Bil will keep the gas good for about six months.
The battery will need to be recharged at least once a month. Any longer interval than that can damage the battery. How you recharge it is completely up to you. Some people make it a monthly action item, others use a battery maintainer (I'm quite fond of the small wall-wart 750mA Battery Tender). Just don't pick up a generic float charger, those can and do overcharge and destroy the battery.
Contrary to some of the posts above, it's better to keep a battery in cold storage (e.g. in an unheated garage) when not used rather than warm storage (like inside a house). There are a few reasons for this. 1) The battery will self-discharge slower, lessening the likelihood that the battery will be damaged from excessive discharging. 2) Batteries give off hydrogen gas when charging. If a nearby spark happens, the battery can explode and you don't want that to be inside where there's a higher probability of people, pets, etc hanging around.
Tires are less somewhat less of a concern in general, but they have been known to go flat and get damaged on cars in storage. If you want to be paranoid, put the car up on jack stands while it's in storage. (This is of course more inconvenient if you suddenly need the car, although this may be an incentive to stick to the bicycle in your case.)