As of patch 3.7, May 2013, the game now officially supports item sets. You can define sets of items in the launcher, and use these in-game at the shop.
You can define multiple sets, multiple categories per set, and any number of items per category.
For each item set, you can specify what champions and maps it appears on, or set them to appear globally.
In-game, you click on the Recommended tab to access a drop-down list which will have all the item sets available to that champion for that map.
If you specify an item set to be specific to a champion, it will default to that item set when you first visit the shop. Once in-game, if you select an item set through the shop, it saves that item set selection in
Config/game.cfg under [RecommendedItems], and the next time you play that champion on that map it will show the item set by default.
The item sets are currently stored client-side in
RADS/projects/lol_air_client/releases/*highestVersionNumber*/deploy/bin/preferences/*username*.properties. If you play on multiple computers, simply copy this file to copy your item sets.
They have plans to move them sever-side in the future, so you will eventually have access to them from any computer. They've also hinted that they may be adding import/export functionality, to share your item sets more easily.