There are different types of stylesheet types you can use. However, I've found it to be useless as IE4 on pocketPC ignores that and goes straight for the stylesheet intended for "screen".
Your second options is to detect the operating system by using CGI variables on the server side, and redirecting to a small-res site.
Additionally, you have the option of using WAP for cellphone site access.
In any case, semantic HTML markup is your friend. If you seperate your style and content properly, then changing your site around using just a stylesheet should be no problem and will only require minimal additional support to implement.