A while back, Cox @Home made it very clear that the use of VPN protocols is
prohibited. AT&T @Home (my provider) hasn't made any similar statements, so
I think it's more of a policy issue for the various cable companies than it
is for @Home themselves.
Of course, there's always httptunnel. Stick your IPSec/PPTP/L2TP traffic
inside good old http and you'll get anywhere you need to go...