Does the .bitcoin directory shrink in bitcoin-qt 0.7.2 and later? The bitcointalk.org thread "Ultraprune merged in mainline" implies that a more efficient way of storing block information was introduced. Will this make the .bitcoin directory smaller after an upgrade to 0.7.2?
The code referred to by that thread will become 0.8, and is not in any released versions as of January 2012. 0.7.2 is a minor upgrade to 0.7.1 only, with only bug fixes.

0.8 will indeed use a more efficient storage mechanism, which will most likely result in slightly reduced storage requirements (though you may need to delete the old database manually). The most important improvement, however, should be block validation speed, and the amount of I/O to disk necessary.

In the future, the new database scheme may lead to a version that supports block pruning, where not (all) history blocks are stored on disk anymore without compromising security (it would still require downloading those blocks in full, however). Because of the invasive effects on the network if suddenly many nodes weren't able to serve history anymore, this may need some discussion first, and isn't implemented yet.