Having developed 4 or 5 iPhone apps now that download remote XML data from servers and populate tables view and other content, I can tell you without reservation that it is the slowest way to go.
Performance using JSON is quite a bit better, but if you can get access to the server to write a Property List (.plist) file instead of (or alongside) the XML you will be much happier with the ease-of-coding and, more importantly, the speed in which the data loads into your app.
If you use mySQL/PHP for your backend, pretty much everything you need to know is explained in this excellent blog post by Aaron Dunnington:
