Diego Giagio has built coded a Linux network driver to allow USB tethering with an unmodified iPhone. The source and more info can be found at: http://giagio.com/wiki/moin.cgi/iPhoneEthernetDriver.
For an easy to install Ubuntu package use this PPA, from pmcenery.
I followed all this and did get it to work once but since then it simply refuses to.
Is there a list of files to clear out and start again with as I suspect a corrupted file thta is not being replaced is the problem.
G
Did you use the PPA or install from source?