I'm interested in the S30 (available in March , I think) but I have a question about the plugin boards. Besides the XG Tone Generator and the Piano Plug-in board, will there be any other boards that add more waveforms and patches to the synth? I like the other boards, specially the idea of adding more synth engines, but I would also like to be able to add more samples. Like an orchestral plug-in board or something.

I own a S80 that is similar (99%) to S30 as you know. The plugins are not just sample ROMs but plug in synthesizer modules with thir own polyphony, insertion effects, editing software etc. Avoid XG100 and DX100 boards as they have some sortcomings. These two boards are made for older MU series modules and not new Sxx. You can buy piano 150PF and 150AN witch is a true analog synth in a board (same as AN1x) with its own computer based editor. Also you can buy the 150-DX board that is a DX7 in a box.

In my S80 i have PLG150PF (piano) and PLG150AN (analog) as most S80 users i know.

Avoid XG plugin due to its poor & thin voices. It sounds as a home toy-keyboard because its parts are assembled from just two sampled elements instead of S80 four elements parts. You need XG plugin only to playback computer MIDI /XG-GM files you have.

PS: S80 sounds really great and the pianos on PLG-150PF are of the best in the market today.

Are those plug-in boardS the same as one for SW1000XG PCI card?

As i know (i don't have an SW1000XG card) only PLG100 type boards are exclusively made for SW1000. The PLG150xx are made for modular synthesis synths & modules but i think that you can use them on SW1000XG.
You can grab more info about them from Yamaha's site.