PPTP is merely two TCP connections between the PPTP server, and the FEP.
The client talks PPP, not PPTP, although as the dreez pointed out, the
client will likely have to indicate somehow that it wants the FEP to
initiate the PPTP communication to a particular PPTP server. So, given
that the FEP site can route, filter, proxy, or do whatever else it likes
to a given port, and that the same would hold true for the PPTP server
site, I'd say the answer is that "yes, its routable".
NT 4.0, which includes a full PPTP server implementation (to the
specification that is currently available...but not necessarily
finished), does not provide any mechanism to route PPTP connections it
receives to another box. Of course you could use Catapult to do this.
And any Firewall with a generic proxy should be able to route incoming
PPTP connection attempts to a specific NTS.
Hope that answers your question.
...eek, quick, someone give me some broken software, I'm suffering beta