Ok I finally figured it out, but haven't solved it really. It turns out that the host I'm using (aplus.net) blocks port 80, and all server-outbound traffic for "security reasons."
Unfortunately, they weren't able to disable that firewall for me, and my fiddling with the mt.cfg proxy settings didn't produce anything of value. So if you want trackback functionality with MT (kindof an important feature imo.) stay AWAY from aplus.net. I still have my 60 days of post-domain registration to serve out at aplus.net, so I'll be using the following workarounds in the meantime: (note: these should work for those having similar problems in their work environments).
Standalone trackback pinger at
http://kalsey.com/tools/trackback/and meta-blog service trackback pinger at
http://pingomatic.com/It's a little extra work, but in 2 months I'll migrate over to Dreamhost or whoever has the best Movable Type friendly reputation, and this pinging snafu will be history!
best,
michaelm of helio7.com