I'm currently importing the blockchain .dat files from http://eu1.bitcoincharts.com/blockchain/ using
bitcoind -loadblock=/var/lib/bitcoin/blockchain/blk0001.dat -loadblock=/var/lib/bitcoin/blockchain/blk0002.dat -detachdb
This has been running for two days (it's a very slow computer), and I noticed that since block 193002, bitcoind 0.7.0.3 is flushing wallet.dat after almost every imported block (before that, there's only one flush after block 225). I'm not sure what's going on here. It's a fresh ~/.bitcoin profile. I think block 193002 comes around the time blk0001.dat filled up and it switched to blk0002.dat, maybe that triggered something in the bitcoin-wallet thread? (There's a comment "// Don't do this if any databases are in use" in walletdb.cpp, but I'm not familiar enough with the code.)