Although Microsoft allowing Rare to produce games for the Wii would be unlikely and unprecidented they could have a deeper purpose behind it; besides the obvious "Microsoft is going third party!" or "Microsoft plans to flood the Wii with crappy games!" conspiracy theories that is ...
One of the problems Microsoft faces when they're trying to make games which appeal to a broader demographic than the XBox or XBox 360 have sold to is these gamers don't own the system to play these games on, and they're unlikely to buy a system to play these games on. To see what I mean, imagine how well a brain training or Nintendogs title would sell on the XBox 360 if they were released today; they would probably be profitable but wouldn't receive the same level of popularity as their Nintendo DS/Wii counterparts.
In the next generation, if Microsoft wants to appeal to the broader demographic they will need several high quality well known franchises to attract to their more affordable and approachable system; some of these franchises can probably be taken from third party publishers that have produced these games for the Wii (or PS2 previously) but Microsoft would (probably) need a few internally developed.
What this means is Microsoft has a problem because they can develop these games internally for the XBox 360 but they (probably) will not become that well know or popular, they can buy third party developers to produce these games for their next generation system but those may not be that high of quality (with how some third party developers approach their "Casual" Wii games), they could try to learn as they go by developing their first wave of these games when they release their next system (but these will obviously not be well known, and there will be a stiff learning curve to make them quality), or they could do the unprecidented thing of developing these games for the Wii and/or Nintendo DS.
Personally, I would doubt we would see any/many games developed by Microsoft for the Wii; and I suspect they would probably try a multi-tiered approach of internal development for the XBox 360, buying popular third party developers, creating new development teams for the launch of their next system, and allowing some developers to explore ideas on the Nintendo DS.
