According to apfeltalk a German site Apple has shipped 8GB iPhone 3GS to a customer by mistake whereas he has placed an order to Apple for an iPhone 3G but when he opened the shipped package that was an iPhone 3GS with 8GB memory.

It means that Apple really has a plan to launch iPhone 3GS 8GB in near future and some evident about this was also published by Rogers in the beginning of August 2009, and they conformed that Apple will soon launch iPhone 3GS 8GB.