<p>Have any well-documented or open source projects targeted <code>iPhone</code>, <code>Blackberry</code>, and <code>Android</code> ? Are there other platforms which are better-suited to such an endeavor ? 
Note that I am particularly asking about client-side software, not web apps, though any information about the difficulties of using web apps across multiple mobile platforms is also interesting.</p>

The iPhone uses Objective C, the Blackberry Java SE with RIM functionality and Android another custom version of Java. I could possibly see how you could combine the latter two but there is no functionality (without jailbreaking) of running Java applications on an iPhone.
The best bet I've seen so far is something like
Qt
that will run on Windows CE, almost certainly shortly Symbian, some Java platforms and the three major desktop OSs.