I also did try Ethernet, but I had the same lockups. I have to explain that the Airport base station I used was the one with two Ethernet ports, on that did do my DSL connection, one Ethernet port for the computers in my home office and wireless for my iBook. Even if I connect my iBook to Ethernet and then used the Internet through the Airport base station as DSL router it did lock up under heavy load.

Currently I am using a Netgear FWAG114 DSL router and dual band wireless access point and it does not lockup, it does not matter if I use LimeWire via the Ethernet or wireless, I have let LimeWire running for weeks and had no problems.