Download Usinagaz !
You'll need the GNU HpTools if you want to recompile it. Online documentation can be found there (both HTML & txt versions).
Beware, XON/XOFF was bugged in Linux PPP, so that usinagaz will work poorly on hp48. You should be able to connect, but not get big data. You'll have to apply this patch to 2.2.x kernels, or this one for 2.4.x kernels, or that one(not tested) for old 2.0.x kernels. 2.6 kernels (starting from 2.6.0-test3) are at last corrected.
Well, there's still some big work to make it stronger, avoid hacks, etc...
Still missing a modem mobile phone with a rs232 connection (I've seen some on comp.protocols.ppp), and connection to the Internet in the subway will be possible !
You can though already try to browse the web using Yoann Desir's navigator. If you don't manage to have a real Internet connection, you can still try it on this site : here is the DIR-obj of this site.
You can also try the Web server Cheyenne, or the rpl server.
Actually, I'll maybe have another project: an Ethernet-style network using IrDA : meeting around a round table for an xblast on hp !
If you have time to loose, you can have a look at the history of Usinagaz