I sometimes use a Novatel "Merlin for Ricochet" PCMCIA wireless modem.
When I dial up, I sometimes see a response "CONNECT 460800", i.e., 
460800 bps.  However, I can only set the speed to a maximum of 115200 
with pppd; the error message implies that it's the driver that's 
objecting.

I haven't been able to figure out what speeds are legal in com.c.  If 
nothing else, it seems to depend on sc_frequency, and that seems to be 
passed in from some external source.  At least as important, I'm not 
sure if it matters, since I don't know what the uart on the card is 
doing with the information.  (We'll leave completely aside the question 
of whether or not Metricom's network can transmit that fast.)

Here's what I see when the card is inserted:

pcmcia0: CIS version PC Card Standard 6.1
pcmcia0: CIS info: Novatel Wireless, Merlin Ricochet Modem, NRM6842, 
pcmcia0: Manufacturer code 0xa4, product 0x1aba
pcmcia0: function 0: serial port, ccr addr 200 mask 3
pcmcia0: function 0, config table entry 32: I/O card; irq mask 10; iomask 3, iospace 3f8-3ff; io8 irqlevel
pcmcia0: function 0, config table entry 33: I/O card; irq mask 8; iomask 3, iospace 2f8-2ff; io8 irqlevel
pcmcia0: function 0, config table entry 34: I/O card; irq mask 10; iomask 3, iospace 3e8-3ef; io8 irqlevel
pcmcia0: function 0, config table entry 35: I/O card; irq mask 8; iomask 3, iospace 2e8-2ef; io8 irqlevel
pcmcia0: function 0, config table entry 36: I/O card; irq mask ffff; iomask 3, iospace 0-7; io8 irqlevel
com3 at pcmcia0 function 0: serial device
com3: ns16550a, working fifo


		--Steve Bellovin, http://www.research.att.com/~smb