Mark
Technically speaking, I don't know whether the Pioneer firewire output is MPEG-2 (the DVD standard), but I can report that I have successfully imported video from the PIoneer to iMovie via firewire. All I know is that somehow between the DVD recorder and iMovie, the signal is converted to DV - simply and easily and requiring no additional software (how good is that!).
Unfortunately, I haven't been able to import to iMovie HD (5.0.2), but it always works with iMovie 4.0.1. I think from memory that version 5 reports "camera no tape" (or similar) and you can't get past that bottleneck, given that the DVD recorder is not a camera, as such. So, I do my importing with version 4.0.1 then use 5.0.2 for editing.
Bear in mind that this import process won't work with commercial DVDs that are copy protected, but is handy if you ever want to bring your own projects back into iMovie where the only source is DVD and no edited master tape is available.
Also, be aware that you will suffer some quality loss using the described method - it's always best to re-import from DV tape (or hard drive or data DVD backups), if available. The conversion from MPEG-2 to DV then back to MPEG-2 on encoding to a new DVD, will result in some loss of quality - but this is probably not that perceptible.
John