That sounds dangerious. The patch/expansion would have to "rip" the other, temp MP version out. Plus it would have to be tailored to culture, UUs, etc. That would be worse than waiting for MP.
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"I agree with everything i've heard you recently say-I hereby applaud Christantine The Great's rapid succession of good calls."-isaac brock
"This has to be one of the most impressive accomplishments in the history of Apolyton, well done Chris"-monkspider (Refering to my Megamix summary)
"You are redoing history by replaying the civs that made history."-Me
The question is if they will "give" an MP patch to all CIv3 SP buyers, or if they will "sell" an MP version + expansion pack, like in Civ 2 "Multiplayer Gold Edition". I´m afraid it could be the second one, but I don´t care if the new MP features are really cool.
To be honest, even though I don't want to wait or have to pay for MP, a retailed MP XP would probably be of a higher quality than just a downloadable patch.