If you want one, can afford one, and can justify it to yourself, then now is always the time to buy one. For one up back country touring, it is a great bike. If you are looking for a "dirt bike with a plate", it is not a great bike. Will BMW be producing a entry level ~ mid level dual sport in the future? Almost a certanty. Will it be better than the Dakar? Maybe, maybe not. There will always be a market for used F650's, as they are a great utility bike with incredible fuel economy (I have gotton as high as 69.8mpg on my GS). So if BMW did start building something bigger, someone will certainly buy your used Dakar.
But, it might be hard to find a Dakar now. They go quick once they hit the dealer floors. Heck, even the standard F650GS's go quick.
So, if you want it, go get it.
1999 Dodge: stock -- noisy, stinky & handles like a bloated pig.
2009 BMW F800GS: fast and sexy.
2000 Husaberg FE600E: Makes KTMs tremble with fear.
2007 Suzuki DR200: The 'girl' bike.
1981 Honda XL250S: olds cool funkiness.
1980 Honda CM400: rat-bike.
"Life is meant to be enjoyed, not endured" ~someone