If someone was to encode as much music as they possibly could onto a DVD, using the standard filesystem and compression methods, how much could they manage? Just music, no images or special features or ultra-secret exclusive bonus information.
By standard, you mean regular CD quality? (44khz@48,000kbps?)
I think re-writable DVDs can store 4.7GB per side (9.4 gigs for a dual-sided DVD), as opposed to 650MB for a regular 74 minute audio CD. So, 9,400mb per DVD/650mb per CD = ~14.4 regular CDs worth of music. At 74 minutes per regular CD, it comes to around 17 hours worth of music.
If you compress the music first to MP3s or Oggs, you could probably fit a weeks worth of music on one DVD. 
That’s not a fixed number because compression algorithms will produce different size files depending on the content of the input. Also common lossy methods such as MP3 allow variable compressability.
NutWrench: Yes, I mean standard CD quality. I mean standard CD everything.
How do rewritable DVDs compare to normal DVDs in terms of storage? 17 hours is freaking amazing, but can regular DVDs do any better?
I might be wrong, but I thought it was 1400 kbps.
You’re right. 44,100 samples per second, each using 16 bits, times 2 channels, equals 1411.2 kbps.
Yup. Double that number. DVD-ROM discs can be dual sided and dual layered. 18GB can be stored on the DS/DL discs.
Also, the first generation of Blu-Ray optical disc recorders have been released in Japan. 23GB on a single recordable disc! It sure will be nice to have one of those babies when the price comes down.
DVD blanks will have roughly the same capacity as a commercial single-layer disc: 4.5GB, aka DVD-5. The discs you get movies on are, however, dual-layer DVD-9 with a capacity of 9GB. In theory, one could get dual-sided DVD-Rs with a total capacity of 9GB, or dual-layer dual-sided pressed DVDs with a total capacity of 18GB.
Oh, and if you’re curious about DVD technology, I’d recommend the excellent DVD Demystified FAQ.
WOW! I remember reading about blue lasers for optical media way back when all the big boys (Phillips, Toshiba, etc) couldnt decide upon a standard for CD-Rs! Any idea what year that was? I think i had a 800 MB HD then. 
Hmm, good question. I wasn’t sure and was curious myself, so I Googled it. I found a 1997cite stating:
" 23GB on a single recordable disc!"
Easier to get a 200gig HD & put music on that. Might be out some day what with MP3 players holding more & more.
Easier, maybe, but certainly not more portable. And certainly not resistant to magnets or getting dropped on the ground.
A DVD-5 (4.7GB) can hold between 6 and 46 hours of CD-quality stereo audio depending on the format and bitrate you choose:
PCM (1536 kbps): 407 mins / 06:47:00
DTS (768 kbps): 815 mins / 13:35:00
AC3 (224 kbps): 2797 mins / 46:37:00
PCM is the only one of those three that’s actually true CD-quality audio. While DTS, AC3, and MP2 at high bitrates can do a great job, and you may be unable to tell the difference, they are still using lossy compression.