1) Supports incremental backup or full backup as determined by me
2) Will back up to DVD, and notify me when it's time to swap DVDs
3) Able to back up both Windows and Linux boxes (if it can do this via SMB, that's perfectly acceptable)
4) Open-source or well-documented file format, to guarantee I can get my data back
5) Not stupidly expensive
Any suggestions? I've completely failed to find anything - most people just say "rsync!" (which fails on #1 and #2), some of the smarter ones say "Amanda!" (which fails on #1, albeit more subtly, and #2), and everyone else just says "get a commercial system!" which, at the very least, fails #4 and #5.
I'm half considering writing my own here, but I'd really rather not.