I used Google video chat over UMTS on my n810 tethered to my N75 very successfully from downtown Manhattan in the middle of the work day. I also used Skype and SIP (grandcentral) successfully. I think the existing infrastructure at least here in NYC could probably handle a small spike in 3G usage for VOIP. It definitely managed the spike after the iphone was introduced.
I, as many will, welcome our HSPA providing overlords into our tablet community.