If it is out of warranty, Apple will replace the battery in your iPod touch for a cost of US$79 plus US$6.95 shipping.

Unfortunately, not many third-party services cover the iPod touch yet. The battery in the original iPod touch is soldered to the logicboard and the battery in the iPod touch (2G) is attached with adhesive. Both are extremely challenging to replace. Professional installation is strongly recommended.

Nevertheless, for very tech savvy individuals, iFixit has instructions for replacing the battery in the original model and MacRecycling has a two part video disassembly series that can be used for the same: