Apple announced in January at Macworld that every iPod touch manufactured after 15h January 2008 would come pre-installed with the 'January Software Update' which contains the iPhone applications (Maps, Notes, Mail etc). To compensate this, many retail stores, under the instruction of Apple, have started to discount the retail price of iPod touch by exactly $20. Apple see it as you then spending this 'saving' of the $20 on the 'January Software Update' anyway.
A note: Please, if you are going to buy an iPod touch, can you 100% make sure that iTunes is supported in your country, and that you are able to make a valid iTunes store account if you do not already have one. Many people have complained after buying an ipod touch that they cannot update their software with the January Software Update, as iTunes does not support their country. Because I do not know which country you reside in from your profile, I thought I'd let you know, so that you could check this before purchasing, because if iTunes isn't supported you won't be able to buy any 'chargeable' software updates from Apple, as they are only available through iTunes. 🙂
R-Fly