Go to Apple and get there at midnight tonight (it's already Wednesday here.)
It's been allover the news that anyone without a pre-order or reserved iPhone 4 will have to wait until June 29th...sorry
The guys at Walmart told me they don't expect to have 32 GB models as they've only ever had 16 GBs before. But, a quick check of their website shows the 3GS in all it's flavors and sizes.
One problem I have - my wife is out of town and I'll have a 6 year old with me, makes it hard to get there at midnight He'd be good for 2-3 hours in the morning and I can have someone pick him up from the line if it takes longer.