If you have avoided the first day iPhone 3G on sale frenzy you did the right thing. Apple's activation servers went down and spoiled the first day iPhone 3G owner experience for many. The servers seem now up again and hopefully stay up during the next days.

Apple publishes each evening the iPhone 3G stock for its Apple Stores throughout the nation and the stock situation looks pretty good for Saturday.

In California only four stores have no iPhone 3G inventory. All San Francisco stores are stocked with the new iPhone. The New York City stores are also all marked with all iPhone 3G models in stock.
More details on the Apple Retail Availability page.

It is not clear if AT&T stores will have stock again. Lots of them sold out on Friday according to Engadget.

In the UK the iPhone 3G is totally sold out. O2 informs on their site that they work with Apple to get on a weekly basis new iPhone 3G phones in and that by the end of Summer all people who want an iPhone 3G will get one.