longer battery life? i don't know. but
http://buyersguide.macrumors.com/ look at the length of time between updates and guesses when to buy or not. the 15/20/40GB were released april '03, so i guess its best to maybe wait a bit if it isn't necessary. the last update was the release of the iPod mini and the bump of base model from 10GB to 15GB (not major), so it might do you well to wait a few months. there might be a price drop, or a new version released. its all up to u and what you want. theres a rumor of a new iPod Ad in productions, and have long been rumors of iPods with color screens for pictures from the card readers, but no real specifics. and Toshiba accidently let it slip that apple will be using its 60GB 1.8'' drive.....
essentially, its up in the air, but its really up to you
edit: 1 more thing.
they're releasing updated iMacs in september, and it isn't unlike apple to release two consumer grade products at once.......