http://www.pocket-lint.co.uk/news/news.phtml/10212/11236/apple-iphone-3g-coming-2008.phtml

When asked why the current model didn't have 3G, Jobs blamed power issues saying that the 3G chipset would be too much of a drain on the unit's battery life which promises 8 hours of call time, but said that future models would have the technology.