Nico Nico Nii!~
This thread contains obsolete information and was last relevant August 27, 2013. See the 2013-2014 thread for the new lists effective as of August 28th, 2013: http://www.pokecommunity.com/showthread.php?t=309435
Time to make a new thread for this, since the new format has been in effect since September 1st.
So here's the 2012-2013 format, as of 06/05/2013:
Black & White
Black & White: Emerging Powers
Black & White: Noble Victories
Black & White: Next Destinies
Black & White: Dark Explorers
Black & White: Dragons Exalted
Black & White: Dragon Vault
Black & White Boundaries Crossed
Black & White: Plasma Storm
Black & White: Plasma Freeze
Black & White Trainer's Kit
Black Star Promos numbered BW001 and higher
Also, anything reprinted in these sets from an older set as legal. That means the following:
For Pokemon, it means it has to be the exact HP, Weakness, Resistance, Retreat Cost, Type, and have the same attacks (the attack text could be altered some by TPCi on a reprint to make it fit better with the way the rules on wordings work in the meta game) as a card in a currently legal set.
For Trainers (now Trainer - Item)/Supporters (now Trainer - Supporter)/Stadiums (now Trainer - Stadium), it has to have the same name and the same effect when played. It doesn't have to be the same Trainer class as it was before, since the way that works is that the older cards are counted as the Trainer class that the reprint is, at least in most cases (for instance, the Trainer from the Base Set, Bill, was reprinted in HeartGold SoulSilver but had its card type changed to Trainer - Supporter, which is what all versions of the card must be treated as; on the flip side, the original Computer Search was banned entirely because of a card type change to Trainer - Ace Spec). Any changes in the card wording always go by the newer version of the card. See this post here (the words to the left are a link) for a partial list of cards that are not allowed to be used despite having a name-sharing reprint. If you know of any other cards, please PM me them and I will credit you with finding this information out for me.
For Energy, it has to be the same name and energy type (Special Energy versus Basic Energy, I mean), though you adopt any of the newer wordings.
There are several reprints that are currently play legal. I may go through and compile a list of which play legal set it was reprinted in, and what other sets it was printed in previously.
Also, any future sets will become legal as they are released, as far as I understand Play!Pokemon's rules for the TCG, which I presume aren't any different from Pokemon Organized Play's rules for the TCG.
And a lengthy, official PDF detailing the rules from Pokemon.com here: