Scores of people who ordered the 2.5 GHz G5 as early as the day of its announcement received emails today from Apple stating that their machines won't ship until or before August 20. The machines were supposed to ship tomorrow.
This seems to be a recurring theme for Apple: Pre-announce a wonderful new product, give a shipping date, and then fail to meet that date (save for a few units here are there).
However, not everyone has received an email notification of a shipping delay. Hopefully at least some of the units do ship tomorrow, and the August 20 date is the worst case scenario for the rest of units on the waiting list.