Final Fantasy XI player still playing the game on the PlayStation 2, it looks like you're in for a disappointment. After the surprising announcement that the game was getting a new boxed expansion, it was noted that the official site didn't list PS2 support. According to community representative Matt "Baryohne" Hilton, that wasn't an accident: Seekers of Adoulin will not be available for PS2 players in North American and European markets.
Japanese players on the console will still be receiving the expansion, likely due to the higher percentage of console subscribers in Japan compared to other regions. Hilton was quick to point out that support would still be offered to PS2 players, but that's cold comfort to any console players left out of the upcoming expansion. There's been no word yet on whether or not the game will begin phasing out console support entirely, but if you consider the age of the PS2, it's entirely within the realm of possibility.
[Thanks to FusionX for the tip!]