You need to get it as soon as you can to get the upgrade eligibility clock rolling.
If you get the phone in June you will be eligible for another upgrade in 12-18-20 months. But if the stick supply is low and you have to wait till sept than this ouches your upgrade cycle back further.
Att may not be as nice next year and won't move peoples eligibility up 6 months again.
That being said, just pre order it. If it's anything like the 2008 3G mess, we are talking middle of August for in store availability. Especially with the white manufacturing problems.