Until last night, that is, when I noticed that any access to the Internet began telling me that I was "not subscribed to any cellular data service".
Setting the APN back to "internet" (or removing it entirely) restored Internet functionality to my phone, but only the broken "you're just going to do web browsing and mail, aren't you" functionality. PPTP is still broken on the "internet" APN.
Two things: first, has the "direct" APN been turned off? Or has a recent plan changed removed access to it? If it has, can it be turned back on?
Secondly, the reason PPTP doesn't work (for me) is that the control TCP connection comes in on a different IP address to that attached to the (GREv2) data packets, so PPTP the server, which isn't expecting that, doesn't know what to do with the packets. Sorry, 2degrees, but that's the most busted-arse NAT behaviour I've seen for a while. PPTP works fine thought NATs (with certain caveats), as long as you don't do braindead things like that.
If you could just use the same public IP address for all traffic coming from the current incarnation of each phone's connection, that would make it work much better. I should imagine that IPSEC/IKE traffic would suffer the same problem, before anyone recommends using a protocol that's only marginally less crap than PPTP.