The difficulty depends on the platform you're coming from / going to. Porting CSI from PC to Xbox 360 was fairly painless because the Xbox 360 and the PC are pretty similar under the hood.* Porting it to the Wii was a lot harder because the Wii is completely different.
Now that Telltale's tools work for Xbox 360 and Wii, the porting process should be somewhat easier for us in the future. But as mentioned, if you're not designing with the console in mind to begin with, it still might not turn out to be a great port.
*I can say that because it was finished up a few months ago and I have a short memory. And also I'm not a programmer.