What is Modified?

In the Modified format (the format for all Pokemon TCG Premier Events), a player may only use cards from Diamond & Pearl and on.

The sets that are legal for DP-on can be found in the set symbols document. (yes there is a typo in there, I know, but the information you need for deck construction is accurate).
60-card deck. Sets from DP and after.

2009-2010 Modified Format

Scared of losing your DP-GE cards in a few months? Fear not! DP-GE will be legal next tourament year in Modified: http://www.go-pokemon.com/op/news/articles/55.html

2008-2009 Modified Format Document Up!

Check out the "What is Modified" section on the website.
It's a document with all the set symbols so it will be perfectly clear what sets will be legal in the Modified Format.

2008-2009 Modified Format

POP announced the next Modified format. Keep in mind that this does not take effect until September 1st, 2008. Still plenty of time to use all of your HP-MD cards!

The sets being rotated out of the Modifed format on September 1st are:
EX Holon Phantoms
EX Crystal Guardians
EX Dragon Frontiers
EX Power Keepers
POP Series 4 Promo Cards
POP Series 5 Promo Cards
All Black Star Promo Cards numbered 037-040

The Pokémon Set Symbols document has already been updated to reflect this.