Uche's blog discusses Streaming XPath. BTW Uche, great hat - makes a nice change from large bushy beards on geek photos :-).
The idea is to be able to perform XPath-like expressions on streams of XML events (like SAX or the forthcoming StAX API).
Also of interest is STX which is like a streaming XSLT which also comes with its own streaming XPath language. There's a Java implementation called Joost.
(It'd be groovy to integrate this with Jelly too :-).