Update: My problem appears to have been the 1GB memory limit for the CrashPlan service on my machine. This means (surprise!) that nothing had actually been backed up for an awfully long time and I hadn't noticed at all.

It's now cranking away at a steady 5Mbps upload, with an estimated completion time of 1.4 days from now.

I guess I could push it to go faster, but this is roughly 1/3 of my total upstream budget, and seems to have no impact on interactive performance. I'm really, really enjoying my upgrade from 15Mbps/1.5Mbps AT&T Uverse to 200Mbps/15Mbps Comcast.