After installing deb 3.1 I want to upgrade to the 2.6 kernel but find i lose dsl connection afterwords. Any suggestions would be appreciated. What man page to read, what kernel patch to install. Should I install via the shell or kpackage, does it make a difference?

Installing 3.1 with the boot command linux26 would have given you a 2.6 kernel to begin with. Not sure what the reason is for your loss of connection but installing packages using apt of kpackage doesn't make that much of a difference. Kpackage is in the end just a GUI for apt (just like synaptic which I prefer to use)

I can think of couple of problems...

If you have more than one network card then most likely eth0 and eth1 have changed places. Network cards are sometimes "seen" in different order with 2.6 kernels. Try reconfiguring your network cards and see if that fixes your problem.

Another possibility is that the kernel module for your network card has several different modes and 2.6 defaults to the wrong one. As far I've seen this problem only on AMD HomePNA cards. They have both Ethernet and HomePNA modes: 2.4 defaults to HomePNA and 2.6 to Ethernet. This could be fixed by passing parameters to the kernel module.

It seems to me that when you have been configurating kernel (menuconfig I suppose) you forgot to select items needed for dsl. Simply reconfigure?