I'd been looking for the ipheth internet interface with ifconfig, but discovered that one had been created with iwconfig - "eth1".
Here's what I get: $ sudo udhcpc -i eth1
dmesg
..
ipheth 1-7:4.2: Apple iPhone USB Ethernet device attached
usbcore: registered new interface driver ipheth
------------[ cut here ]------------
WARNING: at net/sched/sch_generic.c:255 0xc034f714()
Hardware name: Latitude D430
NETDEV WATCHDOG: eth1 (ipheth): transmit queue 0 timed out
Modules linked in: ipheth squashfs scsi_wait_scan ramzswap loop pcmcia yenta_socket sdhci_pci dell_wmi video sdhci rsrc_nonstatic backlight
wmi battery pcmcia_core ac output mmc_core tg3 ssb libphy
Pid: 0, comm: swapper Not tainted 2.6.33.3-tinycore #2012
Call Trace:
[<c0128285>] ? 0xc0128285
[<c034f64d>] ? 0xc034f64d
[<c01282ca>] ? 0xc01282ca
[<c034f714>] ? 0xc034f714
[<c013a661>] ? 0xc013a661
[<c01242b2>] ? 0xc01242b2
[<c01138e2>] ? 0xc01138e2
[<c0113ab5>] ? 0xc0113ab5
[<c01402ff>] ? 0xc01402ff
[<c012faa4>] ? 0xc012faa4
[<c012fbb4>] ? 0xc012fbb4
[<c012c1af>] ? 0xc012c1af
[<c012c252>] ? 0xc012c252
[<c012c5d4>] ? 0xc012c5d4
[<c0114204>] ? 0xc0114204
[<c03bd06a>] ? 0xc03bd06a
[<c026ce02>] ? 0xc026ce02
[<c032a7bd>] ? 0xc032a7bd
[<c01017e9>] ? 0xc01017e9
---[ end trace debb96b4aa274556 ]---
ipheth: ipheth_tx_timeout: TX timeout
As said, internet tethering is blocked on my iphone, but I borrowed a friend's iphone with internet tethering enabled and got the same result.
Maybe you could give it a try?
Edit: as per google, several others have this message. The libimodiledevice devs are saying that, since the ipheth driver is accepted into the kernel as from 2.6.34, any problems like this need to be posted on the kernel mailing lists. It's almost certain that we would be told to upgrade to the latest stable kernel if we post this issue on the kernel mailing list...