This refers to the reference Bitcoin client (bitcoin-qt), version 0.7.2. While it was catching up with the blockchain, it did cause such a high I/O waitstate load that working on the same computer alongside was very frustrating to impossible.
Setting bitcoin-qt to low CPU and IO priority helped a bit, but not too much:
ionice -c 3 -p $(pidof -s $(which bitcoin-qt))
renice -n 20 -p $(pidof -s $(which bitcoin-qt))
Additionally setting the main application(s) you work with to higher prioritoes helps more, but it won't be great still:
sudo ionice -c 2 -n 0 -p $(pidof -s firefox)
sudo renice -n -10 -p $(pidof -s firefox)
There are also alternative solutions for some situations like downloading the blockchain directly.