I bet my weight in pants it going to be on the PS2. It's too early to go Next Gen for the Rider Game Franchise, with the PS3 collecting dust, and the 360 still being too pricey/slight unappealing to Japanese gamers (The likes of Blue Dragon has helped greatly though.) And it would take too much time for Bandai Namco to figure out how to set up the controls for a Rider game on the Wii.
Portable is a possibliity, but it also is quite a downgrade to the TV to a Portable screen.
What I'm more curious is how the Gameplay is going to be, Den-O doesn't lead itself to a fighting system as well as past KR games have. Will it go in the more beat em up game play style of GARO and Kamen Rider Kabuto? Only time will tell apparently.