From WebOS Internals
Regarding this Tethering article: It says: "Once there is a version of the Pre available for a carrier that does allow tethering, or an unlocked GSM version, we may be able to change this policy."
In Germany the phone company O2 (o2online.de) officially distributes the Palm Pre, and it explicitly says that it has no problems with Tethering: http://www.o2online.de/nw/support/mobilfunk/surfmobil/pclaptop/handymodem/handy-als-modem.html (sorry, german language)
And btw: At O2 it's unlocked as well. O2 officially distributes the Palm Pre, but it doesn't SIM-lock it.