If they have a dial-up they definitely should be converting the mdb to txt before they up-load. They can reduce their file size to 5-20% (depending if they compress).
The other benefit of having the your client convert to txt before they ship, is that when you migrate to linux you won't have this lump of MDB sitting there smelling the joint up!
I'm just starting on PHP, but I suspect that a program like Delphi (using ADO) or C++ will be doing the job much faster, than PHP (comments!). I've used Delphi 5 to do a similar type job and it is fairly painless (just stay away from the BDE). You can also write a script in Access to export all the tables to txt. So, if they want fast feedback on a large mdb you should create a utility on there side to as much work as possible.
Hope this helps
Roger