This caveat may be out of date – you can check yourself.

If you are using Netatalk version 2.0.5 or better, this has the special new features added to avoid Time Machine disk corruption. If you have a version of Netatalk earlier than 2.0.5 (e.g. Ubuntu 9.10 currently has 2.0.4), then you are at risk as described.

If you have Netatalk 2.0.5 installed, you should add the option “tm” to any share you use for Time Machine, in the AppleVolumes.default config file.

Non-Apple AFP can be dangerous with time machine…
Authored by: dbs on Thu, Sep 24 2009 at 11:58PM PDT

Everyone should be aware that using Time Machine on a Non-Apple AFP volume via these hints
may put their backup data at risk. Here’s why:

The technical reason why Apple limits Time Machine to 10.5+ AFP volumes appears to be to
prevent disk image corruption. There were additional features added to AFP in 10.5 to
support Time Machine. These presumably allow the disk image engine to force disk image
journal data to write out all the way to the disk. Without such features, a network
interruption can result in a corrupted filesystem on the disk image despite journaling.
Remember, journaling relies on the journal being written all the way to disk before the
changes take place. If you can’t guarantee that (e.g., because of network/NAS buffering)
then the journal is useless. Time Machine appears to rely heavily on disk journaling to deal
with network drop-outs, interrupted backups, and the like. Take this away and your data is
at risk.

If the NAS you are using supports these features it should report them to the OS and you should natively be able to choose that volume. If you have to trick the OS to use the volume it means the NAS does not support it. That trick usually involves the following one long command line:

sudo defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1

So, the safer way to do this is to setup a 10.5 or 10.6 box and share via AFP.