I haven't heard of anything like voip blocking by SBC. I think that would cause quite an uproar. DSLreports.com usually has the latest on such things, so I would say - no blocking by any majors in US.
I've got Vonage (and another voip, pay-as-you-go) on Verizon at 3000/786 bps. Everything is working as advertised.
Of course, DSL ala cart is usually not offered, so most DSL users already have their primary phone connection packaged in and therefor voip is more often a supplement for long distance, extra line, etc. which make things less critical.
Anyway, your voip over DSL should work fine.