Well, I got a proof of concept iPhone application working today. It's not ready to show off yet, as it's little more than a command-line application running in an iPhone window, but I was able to authenticate against a Tririga web service, pass in a username and password, then parse the results.
The iPhone does not have support for NSXML, unfortunately, so we're stuck using the procedural libxml api. Using libxml inside of an OO environment is like pulling teeth. It's a great procedural API, but compared to what you can do with a good high-level language, it's just downright ugly. I'm sure the performance is much better than NSXML, but the code it takes to use it is really, really ugly looking.
It took me longer than I expected, but I've now got the hang of using libXML, and have written a few utility functions to make my life easier. I may try wrapping it up into a low-overhead objective-C wrapper. If I do, I'll post the results here.
In the next day or so, I hope to have a full proof-of-concept iPhone app to submit a work request through Tririga.