I put in my order for the ATI 4870 upgrade kit for the 2008 Mac Pro back when they went on sale in March. Initially the estimate showed it shipping in mid may, but on Tuesday I got the shipping notice. The card arrived today, as well as a phone call from Apple explaining it would not work without software (likely drivers) that they have not yet released, but will soon. Once whatever software is out, they are going to e-mail me about it.
I guess they didn't quite have everything in order to ship the cards when they were ready. My guess is the drivers for the card are in 10.5.7, and someone thought it would be ready this week. I'll be curious to see if the software they called about is just a driver that works on 10.5.6 similar to the driver on the new Mac Pros, or if it is 10.5.7.
In the end it doesn't matter too much that it shipped this week, since I'm on vacation and can't install the card anyhow.