fast roaming for real time apps

[Vieri]

Hi,

Sorry for the novice question but I'm wondering how I can optimize roaming: I would like to switch from one access point 
to another as fast as possible so that real-time applications such as VoIP don't suffer too much.

I know that the client driver and supplicant play a crucial role and an Intel wireless NIC is much more "aggressive" than any other brand.

However, I would like to know if there's a "poor man's" alternative to what Cisco calls Wireless Domain Service (WDS: http://www.cisco.com/en/US/tech/tk722/tk809/technologies_q_and_a_item09186a00804d4421.shtml). 
Basically, all I need is to cache the user credentials so that authentication is done only once, thus making roaming very fast even when using EAP-TLS or EAP-PEAP/MSCHAPv2.

So supposing I build custom access points that use hostapd (such as pfSense systems, etc.), is there an open source solution to the user credential caching and fast roaming problem?

Thanks,

Vieri