According to Apple, user removable batteries, such as in the aluminum MBP and white MacBooks (prior to the unibody) are supported for one year or 300 charge cycles, whichever comes first. AppleCare, although it covers most other internal components, does not cover these batteries past the 1 year period. (It also doesn't cover cosmetic plastics and non-electronic or non-moving parts as a rule.)

Batteries in the Unibody models, which are not considered user replaceable, have a life expectancy of three years or 1000 charge cycles. However, if the unit has not been covered by AppleCare, the battery will not be covered beyond the 1 year limited warranty period.

That being said, Apple leaves some decisions to the discretion of the lead Genius or store management. My old store was probably more sympathetic to folks with bulging batteries than other stores, but it was never a definite yes or no.

You also have the recourse of calling Apple Customer Relations. It's their job to handle customer concerns about Apple products, and work as a liaison between the customer and Apple Retail.

Good point above, that swollen batteries are not normal, even if they are fairly common. Apple has said in the past that swollen batteries pose no danger, and should be treated as any other battery - determine if in warranty or out of warranty and process normally. However, because of the damage a swollen battery can do to a computer case, trackpad, or other internal components, I would press the issue with Customer Relations request a replacement. Worst they can do is say "No."