As predicted, Maxis have finally added a page for iPhone 3GS. Go here and you can subscribe for updates via mobile/e-mail too. Price and other details will be announced on the 15th of July. Hope it will come out on the 19th of July as mentioned in WWDC.
There are also rumors that it will be out on 31st July. Having said that, the sooner the better! Come back here for more updates!