libxml2 loads whole xml file into memory.

http://code.google.com/p/touchcode/wiki/TouchXML
http://code.google.com/p/kissxml/

iPhone SDK Development by Bill Dudney and Chris Adamson

NSXMLParser
initWithContentsOfURL: may block GUI while downloading, can not handle HTTP authentication.
initWithData: accumulate data with NSURLConnection’s delegate -didReceiveResponse, -didReceiveData, -connectionDidFinishLoading.

TouchXML
- copy TouchXML files to project
- in “Targets”, select target > Get Info > add /usr/include/libxml2 to Header Search Paths
- add libxml2.dylib to “Frameworks”

UPDATE
instead of adding libxml2.dylib link to the Framework on the system:
Other Linker Flags:

-lxml2

Linking libxml2 in Xcode