i had comunication with Xacd.com when i looked into it - they were very helpful but its best if you have a clear idea about what you want 1st as they wont point out if they think you've got some thing wrong. Things like double butted or tripple butted tubes are a cost option, as are bends in the top and down tube, wishbone seat stays, dropout design etc. Try to get a clear idea about what you want before getting them to draw it up in CAD and give them as much info as possible.
I never got my frame made - as the design i wanted went up to around £750 (delivered). i got tehm to do two drawings though...
It went from this:
to this:

i wanted a 16" frame with low standover, clearance for a 100mm fork, and a long toptube at 584mm (the equivelent of an 18"). Changes i made to the drawings are the bend in the down tube (added fork clearance) and the seat/topetube bridge as it was cheaper than having the tube bent.