Is there an official source regarding this current policy?

@Hanafuda: Aside from Iwatas and Miyamotos public statements? No.
I'm not going to do research for you, you need to look the statements by yourself, or maybe someone is so kind to link some of the statements.
But the reason why Nintendo isn't hyping the games early, is because Nintendo already sell all the hardware they manufacture, so there's no need to promote hardware with upcoming games and most of the game sales are made outside the first weeks. Not announcing games early, leaves you a lot of aces to your sleeve, if the hardware sales starts to decline, they start announcing games earlier and it will boost sales again.
Just look at when SMG, MP3 and Brawl were announced, 1-2 years prior to release to boost the hardware before Wii was launced. Expect Zelda Wii being announced in the way oh yeah, it's coming out tomorrow (although, it was announced being in production in early 2007).

Sorry, but that makes no sense at all.

If all they are interested in is selling hardware, and they know Captain Rainbow isn't going to do that, then why even bother releasing it? 

Also, only leaving a few weeks before announcing a game and then releasing it smacks me as either being incredibly stupid or incredibly cocky.  In Wario Land's case it may be clawing its way to the 100K mark, but Captian Rainbow isn't going to be so lucky.  In which case one can only summise that the strategy has failed.

Also, Wario Land has probably only done as well as it has because practically nothing else has been released for the Wii in Japan for months...

@Hanafuda: No, it makes perfect sense. How early have Nintendo announced its DS games? How have they eventually done?
Of course, if the current policy causes a lot lost software sales, they will change it as easilly as they started it.

And no, the purpose isn't to sell hardware. As i said, most of the game sales are made after the first few weeks (which will be the case with Captain Rainbow too), so the logic Nintendo is using is if it sells a million, it doesn't matter when it does it. Basically steady game sales guarantee longer shelf life in the retail, while frontloaded games die quickly. Now, i'm not saying that this would be case with Captain Rainbow, but as a low developement budget game, Nintendo propably isn't expecting high sales numbers anyway. Of course, this doesn't mean that the game would be bad, it just lacks the final polish.

Captain Rainbow could propably be compared to Wario Ware, in terms of trying something new out, and Captain Rainbow may be just as a fan service or test a new consept (i haven't played the game, but it may have something in its core, thats acceptance Nintendo is trying out). Or then Nintendo is just making the game to build Wiis lineup, or maybe Nintendo is just testing out how their policy works and it's cheaper to do with a game that isn't expected to sell much anyway. Outside guessing it's really hard to say what specific reasons lead to Captain Rainbow and decisions behind it.

^^^This is probably the closest explanation I would give. @ OP what exactly WERE you expecting from Capt'n Rainbow?

...I think the ad strategy my Nintendo is hit or miss being too early to call. ...With Mario Super Sluggers they did the same and it garnered 4th place this week in the US. It looks like they intend to rectify the need for interweb "buzz" by advertising. I don't think Warioland gaining sales is due to lack of games available as sales show other Wii games sale as well. I'm quite sure Warioland once released here will get a good ad campaign as well.

This is just an extension of Nintendo giving odd ideas a chance. Anyone recall Odama? Pinball and strategy fans don't necessarily intersect very often and yet Nintendo gave that the green light too. Be thankful they don't make every release fit into that one mentality of "Games made for everyone" and some titles are there for a vey specific, yet small audience.

^But why release games that are new concepts if no one will know anything about them.

I'm praying right now that Disaster will get a proper ad campaign.

The subject matter of Captain Rainbow probably kept Nintendo from wanting to heavily promote it. I could be wrong though. Nintendo tends to not promote any of their less accessible games or more Mature themed games. Eternal Darkness, Geist, Chibi Robo, Odama, Custom Robo, Captain Rainbow. They even shat all over putting marketing power behind Conker's Bad Fur Day back in the old days. The market a game like Captain Rainbow has probably doesn't need much of a marketing campaign for the game to find it's intended audience. The full page ads in Famitsu were marketing enough. I don't think a "Wii would like to play" campaign would have done wonders for sales.

Is this Nintendo's Segagaga?