Has anyone used WiiFlash for developing Flash games? Any pointers on how to begin developing games for the Opera channel for the Wii?
The WiiFlash project was for using the wiimote on PC/Mac games. I think you want to look at the MarioToo project for creating wii browser games. To be honest though, why bother? You've got a much bigger audience to target with Flash by going for the general PC/Mac/Linux audience.
Keep in mind that the Internet Channel (Opera browser) uses Adobe Flash Lite 3.1; Basically Flash 8 with some Flash 9 features.
I would perhaps target the Wii but make the game accessible to any browser. Best of both worlds. Think of it more like a regular browser on an ancient machine using the Flash 8 plugin and you shouldn't have too much trouble.
I've never used WiiFlash but it might be interesting for testing locally; If you have a Wii already you'd be better off setting up a local webserver and pointing the Wii browser to that local address for testing.
If you were ever going to do a 3D game you'd have to drop Flash entirely and go with Unity3D or some similar engine.
As @Iain mentioned, that is not why the WiiFlash was made.
You will be using actionscript 2.0 to make Flash Games for the WII. There is a title out there on this subject:Nintendo Wii Flash Game Creator's Guide
For Wii games, you might want to check out Unity. It's pretty easy to work with and has portability to the Wii that you can buy.