Jimmy,
I think a combined approach is the most promising:
1) a single XML exchange format
2) multiple wikitext standards based on families of existing wiki syntaxes sharing the largest similarities.
For example, several wikis use a "UseMod-like" syntax, and it would not be very difficult to standardize within that group. It would be much more difficult to get, say, both PhpWiki and MediaWiki to use a shared syntax. The practical difficulties with switching an entire wiki engine to a new syntax should not be underestimated.
Once we have these group standards sorted out, we could try to merge them further.
All best,
Erik