It is my understanding that the nodes save multiple copies of the block-chain in case of a split or one of the block-chains becomes the "longest" one. I have had a test-node running since June 9, 2011 (0.2.22 and 0.2.23) for a total of 55 days. It ran out of disk space today; consuming 5.8 GB. That works out to 105MB per day. Disk usage dropped to 4.9GB when the client exited. The client had 125 connections during peak times.
I have no idea about the test network, but my entire %appdata%\bitcoin dir has never went over 800mb, yet, but i would assume it will be by the end of next month at most.