Now that I have a new MacBook I can repurpose my old MacBook because it still has some life left in it. Surprisingly my little girl's not interested in it—she's content with her iPad and doesn't see a need for a (relatively big) laptop. So, it will make a great upgrade for the G4 Mac mini that's currently serving as our home server for Time Machine backups and shared music.
The Time Machine backups at the house have been a bit pokey, but I wasn't sure where the bottleneck was. Looking at the CPU utilization with Activity Monitor while other machines were backing up to the external drive attached to the mini, the CPU was busy (60–70% utilization), but not maxed out. So, it was probably either the network speed or the Firewire 400 connection to the drive. The MacBook has gigabit Ethernet and Firewire 800 interfaces, so both would speed up and help Time Machine performance.
However, as I was prepping the old MacBook I noticed that the Ethernet interface was connected to my gigabit hub at only 100 MB. I tried forcing a gigabit connection, but the link would drop right away if I did that. So, I tried a couple different known-good cables. Still no dice. Then I connected it to the Airport Extreme's gigabit hub using the known-good cables, and still no luck. OK, let's nuke 'em from orbit: I reset the PRAM and re-installed Lion from scratch. Even that didn't solve the problem!
Some Googling turns up that this seems to be a problem with a small number of minis and MacBooks, and the only way to fix it is to replace the logic board. Of course my AppleCare warranty is expired, and it doesn't make financial sense to pay for a new logic board at this time—I'd be much better off just buying a new Mac mini. So, it looks like I'm going to have to live with compromised network performance.
Looking at the backup times over the last couple days, at least the performance is better. Backups are taking about half as long as they used to, down to about a couple minutes in most cases. But I'm still bummed that I can't take advantage of gigabit network speeds.