What, if any, support is there for CSS3 in mobile browsers? The specific browsers I'm curious about are the ones used in the iPhone, Android, and Blackberry phones. Any links to relevant resources would also be greatly appreciated.
You can try acid3.acidtests.org and http://www.css3.info/selectors-test/test.html on the respective browsers to check some compatibility, but that may not be an exact determining factor of full compatibility. However I don't think any of the mobile browsers currently fully support CSS3.
Both iPhone and Android systems use WebKit as the rendering engine in their mobile browsers. I believe Blackberry are moving to Webkit as well at some point. This engine has some of the best support for parts of CSS 3 available at the moment, as well as quite a lot of proprietary extensions.
I would recommend researching what is available in WebKit, and then testing.
A great resource for support tables is http://www.quirksmode.org where PPK is doing more and more mobile browser testing to answer just these kind of questions.
a do quirksmode.org specifically this page: http://www.quirksmode.org/webkit.html
In the BlackBerry Documentation for Developers, there is a documentation for the BlackBerry Browser, including CSS Reference - BlackBerry Browser. There is no specific mention of CSS3, but that document lists supported CSS properties.
There is also a BlackBerry Widget web standards support page that states 4.7.1 and 5.0 have partial support for CSS 3 color and full support for CSS 3 marquee, CSS 3 media queries, CSS 3 namespaces and CSS 3 selectors.