There is an XHTML mobile doctype which defines a subset of regular XHTML for those devices that can’t handle full XHTML. However, although I won’t claim any inerrability I have heard that most devices nowadays can indeed handle full XHTML so you won’t run into major problems. But I guess how a certain device is performing depends on how complex your website is. In any case it is important to provide proper alternative content, or use the approach of progressive enhancement
for your site(s) to provide at least basic functionality without compromising the information.
For example, the iPhone doesn’t support Flash so an alternative is mandatory. Also, many smart phones such as the aforementioned don’t pass themselves off as handheld devices, so a media type of “handheld” won’t help here, they just use the screen stylesheet (because they claim to display “the real internet”). There are ways to adress these devices with CSS3 media queries
as described on Put Your Content in my Pocket
(and there’s a follow-up article
I hope that helps.