Yes, I use a Hava (Slingbox competitor) and it works fine. It doesn't really matter if you have FiOS, Comcast, or any other provider though (or none at all). All a Slingbox or similar device does is take a video input and stream it over a home network or the internet, so it really doesn't matter where it comes from (cable box, xbox, dvd player, etc.).
Whether or not it "works" well on your computer or your iPhone doesn't have anything to do with FiOS. It just has to do with the quality of the Slingbox/Hava device you have and the upload speed of your home network/download speed of your viewing network.
The only issue that could crop up is the connection between the Slingbox and the Motorola FiOS STB. I think the newest Sling and the soon-to-be Hava use HDMI, which could lead to handshake issues. I personally use component with my Hava, which for some reason leads my STB to output the video as if it was going to a 4:3 TV, letterboxing all HD channels. But if I turn on my HDTV, which is simultaneously connected to the STB, then the HDMI from the HDTV leads the STB to output all its video in native 16:9. Basically that means I have to leave my TV on in order to get 16:9 on my Hava, but that's just a minor glitch you probably won't find with a Sling.