A huge problem in 10.5 is starting multiple networked backups over a relatively slow network -- things just take forever.
To alliviate this, first mount the backup disk on each machine via AFP. Choose that disk in the Time Machine System Preferences panel, start a backup, and then stop it right after it starts. You will have to wait while it creates a sparsebundle over the network. Repeat with each machine.
Once the sparsebundles for all the machines you want to backup have been created, you can bring the external drive to each machine for the initial backup, and it will automatically use that sparsebundle. Once you've done that for each machine, put the drive back on the network, and enjoy not having to wait 10 days to back up over your slow wireless.
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