I've tried one more approach and had more luck with it that I have with any of the others so far.
I got XmBlackBerry to compile, after sorting out some OpenMotif issues. It opens a port in /dev/pts which connects to the GPRS modem. I can actually talk to this modem, and the ATI command identifies it identically to what I see in Windows.
I then try to connect with the same commands as well:
AT+CGDCONT=1,"IP","wap.voicestream.com"
ATDT*99***1#
These work under Windows. Under Linux, they produce a "CONNECT" message, but then the actual PPP negotiation fails:
Connect: ppp0 <--> /dev/pts/6
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xe0267202> <accomp>]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xe0267202> <accomp>]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xe0267202> <accomp>]
LCP: timeout sending Config-Requests
No LCP frames are ever received back.
Update: I compiled the latest XmBlackBerry from CVS, and it seems to include a workaround for this problem. I am now trying to figure out how to get pppd to send a BLANK username/password pair, which it doesn't seem to want to do - it interprets that to mean that it should refuse to negotiate for PAP. Sadly, the remote side insists on having PAP, and the correct username/password for my carrier is BLANK...