According to ModMyi, iPhone 4 will ONLY be available in black for pre-orders and launch. The initial lineup will be the 3G[S] in 8GB, and the black iPhone 4 at 16GB and 32GB.
The white 16GB and 32GB iPhone 4 will be available “later this summer.” You can pre-order your iPhone 4 tomorrow (specific time has not been announced, but the iPad started being available on apple.com around 3-4AM PST when it was available for pre-order).
Of course this is only through AT&T, but it doesn’t seem Apple would short-change their partner in distributing the devices – no official word from Apple on the release of the iPhone 4 yet other than what we’ve already released – launch the 24th, pre-order the 15th.