Heh, thanks for the (belated) replies
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I guess it is a moot point now because since I updated the NAS operating system from DSM2.2-1042 (which it came with on the install CD and which defaulted to ext3) to DSM3.0-1372, the only choice is ext4 anyway. Firmware is not (easily) downgradable on Synology units so I won't bother trying to get back to ext3, anyway Synology must have enough confidence in ext4 to make it the default.
Edit: You still have a choice between ext3 and ext4 format for external drives (eSATA and USB)