I need to display different information to an iPhone user, as they cant display flash. So therefore display a jpeg for example for anyone using an iPhone, and any other device can see the flash content.

I assume the best way to go about this would be to use either JavaScript or jQuery, using the http header information. However I don't know where to start, does anyone have any ideas?

Ideally, I don't want to replace it an error if the person doesn't have JavaScript, as I don't want the iPhone user to think they are missing out.