Good thing is that you don't even have to install. Free for non-commercial use.
It is in free while it is in Beta and even later it should remain affordable.
Supports Flash, Java and ActiveX modes, so your platform concerns are addressed.
The iChat integration I'm not so sure about either. But you don't need iChat to use screen sharing.
I also don't know if it's cross-platform.
That being said, it's a great piece of software; I've been very pleased with it.
Mikogo is great and works seamlessly with Skype (but only on the Windows side). We use this internally; launching Parallels is a small price to pay to be able to be on the same page without having to be in the same room.
I've also used Glance: http://glance.net/site/Home.asp and it works - basically VNC but easier to use I think.
It is a free Windows only application. What I like about that the only software the client needs is a web browser. No ActiveX, no java, no flash. The application is basically a http server that serves screenshots of whatever window has focus. So it does not allow remote control.
It also supports video cams and audio.
Bonus points to anyone that create an opensource multiplatform clone.