4G should
I placed a link in my post check it out. they are using an EVO but it is on Sprint 4G and they are demonstrating Sprint's 4G simultaneous voice/data capabilities. 3G cannot do this.
here is another article, http://www.androidcentral.com/sprint-clarifies-voice-data-evo-4g
I'm not entirely sure that simultaneous use is limited to being on 4G. I was able to "multitask" phone calls and web browsing on my Palm Pre, and I think I've been able to on this Epic. Heck, even the bulletpoint specs on the Samsung Transform (looking at getting that one for the kiddo) show "Android-powered Smartphone With Simultaneous Voice and Data Capability", and its not 4G.
Did you have WiFi on when you were multitasking? That would allow 3G voice and data.
So basically, its not because you are on 4G that you can do data/voice simultaneously, but that you have TWO radios to utilize?