There's a new cdfs on sources that also knows how to read and write data tracks on DVDs and BDs (Blu-ray Discs, the makers claim); see cdfs(4) for details. I've tried to test all combinations of media (CD, DVD-, DVD+, BD) × (-ROM, -R, -RW) × (single-layer, dual-layer), which works out to 24 combinations, but may have missed some (definitely missed dual-density CDs). DVD-RAM is untested but might just work; the rewritable media are much less troublesome than the write-once media. HD DVD is untested; it might just work, but it's fading rapidly.
I've tested PATA (IDE) and SATA burners, but not USB, which ought to work (slowly for now) via usbdisk, nor SCSI. If you haven't been paying attention to optical media lately, dual-layer BDs can hold 50GB, which is enough to be interesting for backups and archiving. There's no officially-sanctioned way (yet) to incrementally add tracks to a disc over a longish interval, but empirically it appears that just not removing the /mnt/cd/wd file until you've written the last track will allow dribbling tracks out to disc. 100 tracks, each containing a 512MB venti arena, should roughly fill a dual-layer BD. It's also possible that I'll implement packet (incremental) writing.
