Battle.net - English Forums -> Post BlizzCon: StarCraft II Trilogy FAQ
Specifically:
How will the expansion sets impact multiplayer gameplay?
The expansion sets will add new content to each race for use in multiplayer matches. This could include additions such as new units, abilities, and structures, along with new maps and Battle.net updates.
If I buy StarCraft II but don’t buy any of the expansion sets, will I still be able to play online?
Yes. This will work similarly to Warcraft III and the original StarCraft, which maintained separate online gaming lobbies and ladders for expansion set players and players with the base Warcraft III or StarCraft.
EDIT: Upon rereading, if you want to be -very, very- optimistic, I suppose they never explicitly said you can't -game- together in that last question (thought that's heavily implied by the 'similar to blah'). Perhaps they will have an option for the Z or P versions to 'downgrade' and play at the other expansion level. I highly doubt this though. It would be nice.
As an additional note, if all you want these games for is the singleplayer content, I would definitely wait for the Battle Chest offer. If you want multiplayer I'd hop in right away because the customs are going to be ROCKING. I've already begun development on some of the maps I'm going to be coding.