The following is a Twitter question/answer session with an EA Sports Community Manager...
Q: Will NBA Jam have an online multiplayer mode on the Wii? If so, can 2 players play online at the same time, on the same Wii? Thanks.
EA: not on the Wii. It will on XBOX 360 and PS3
Doesn't get anymore cut-and-dry than that. Looks like we'll be keeping our multiplayer action confined to the local option.