I'm wondering if and how many third party developpers are currently working on games that will use the Wii Motion Plus - If they are, I think there's probably a "gag-order" placed by Nintendo on news about those games until they release the peripheral.

Do you think that once Wii Sports Resort comes out, there will be a downpour of other titles announced that will use Wii Motion Plus?

HELL yah!
You can bet your dog on that...

All games will have a new face with wii motion plus enhancements.

All I want is a realistic sword fight and boxing game... Nothing more.

We know of a few:

The Conduit
EA Sports Tennis
Red Steel 2
Tiger Woods 10 (hinted at but not confirmed)

I expect Nintendo to make a big deal out of WM+ games coming out at E3

Others that you can expect: Mario Party 9, Rayman Rabbids 4, Warioware, other sport games, other party games.   I question how well Nintendo will support it itself though, because as of yet their barely supported their own balance board.

I wouldn't be surprised that was the reason nintendo themselves have been holding off remaking "Kid Icarus"

Probably more than the Balance Board - which is saying something.

a lot

Boxing would only work if you could use two wii remotes and two M+ since the nunchuck doesn't have M+.

We already know at least four games that are using it, three of which are third-party. I really wouldn't be surprised to hear about a new Zelda which makes use of it for epic sword fights, either.

I actually think that the later half of 2008 seemed so empty from Nintendo was because they want to use Motion Plus in a lot of their core titles.

Dozens of released or announced games support the balance board - it may be as high as 100 before all is said and done. I expect similar things for Wii Motion Plus although since it is coming out some many months after Wii Fit (released in 2007), the total number of games will be less.