The Wii and the DS are following a similar pattern when it comes to third party support at the same point in their lifespan; in ways the Wii is doing better because it seems like more companies are identifying it as the market leader far earlier, but the longer development time of the Wii balances that out.
Unlike previous generations, both the Nintendo DS and Wii have noticeable lag between the time it becomes clear that they are/will be the market leader and when third party support is at a level appropriate for a console that is as dominant as these consoles are. The main reason for this lag is that both of these systems are drastically difference in performance capabilities in comparison to their competition and, unlike previous generations, you can't simply change the target platform in the middle of development of most games.
What we see from the Wii for the rest of the year is (roughly) 150 games with solid release dates, and (as can be seen here http://www.vgchartz.com/forum/thread.php?id=34958 ) approximately 50 of those are third party games that are of fairly good quality. When you contrast that with the number of games being delivered to HD consoles (about 100) and the number of games that were released on the Gamecube or N64 in their life (about 450) it becomes clear that the Wii is seeing much more third party support than recent Nintendo home consoles.
Now, it is time to look back at when these third party games would have been started ... Being that it takes 12 to 24 months to complete most Wii games a game that is being released in the second half of 2008 would (most likely) have started development between July 1st 2006 and December 31st 2007; realistically, the larger and better games would (typically) need more time to develop and would (mostly) have started between July 1st 2006 and July 1st 2007. In this timeframe most analysts were still giving awful analysis that the Wii was a fad, all gamers wanted HD content, and a $400 PS3 would be able to kill the Wii; only recently, after their expected death of the Wii with GTA 4 failed, did most analysts start accepting the Wii as the new market leader.
Inspite of what some fanboys thing, there is no secret society of third party publishers who have meetings about what platforms to support and whether they should focus on HD content; most publishers would come to terms with the new market reality at their own pace, determine to what extent they would support the Wii, and begin executing their own plan. What we have seen from this is that publishers like EA, Sega and Capcom seem to have decided far earlier than other publishers to put decent projects on the Wii (potentially, not that long after Christmas 2006) while other publishers will probably have taken much longer (after GTA 4), or the Wii may not be part of their plan at all (Epic comes to mind).
My personal expectation is that the Wii's third party support will grow in both number and quality of game being released, and a couple of years from now people will look at the second half line-up of 2008 (which, in terms of third party support, is one of the better line-ups a Nintendo home console has had) and try to remember how the Wii's third party support was weak at one point in time.
