Wii U dev kits usually cost around $5,000, but it would appear that Nintendo is forgoing that fee in order to kick-start interest in the system amongst the development community.
The rumour has been triggered by Destructoid's Jonathan Holmes, who tweeted the following:
Hearing more and more stories of Nintendo giving developers free Wii U dev kits. I hope devs take advantage of that. Could lead to victories— Jonathan Holmes (@TronKnotts) April 5, 2013
More support from devs would certainly help the system at the moment. Hopefully this will lead to some interesting indie projects for the eShop, as well as full-price retail releases.