ISI specifications for Nokia modems.
A serious limitation of the N900 connectivity subsystem IMHO is the inability to create multiple connections. One can only have one connection at a time. This was a problem when I started investigating MMS support for N900.
Of course one can use some undocumented API like fMMS to have more than one GPRS connection. There's only 1 problem with that approach. You can end up with IP/subnet clash.