Latest Answer: Upgrade to 1.3.2
Here are various tests I ran via FireBug on http://docs.jquery.com/Downloading_jQuery
Different jQuery versions are switched in at page-load with special greasemonkey scripts.
>>> jQuery.prototype.jquery
"1.3.2"
>>> jQuery(":text[value='']")
[input#jq-primarySearch]
Unknown pseudo-class or pseudo-element 'text'.
>>> jQuery(":text[value=]").get()
[input#jq-primarySearch]
>>> jQuery.prototype.jquery
"1.3.1"
>>> jQuery(":text[value='']")
Syntax error, unrecognized expression: value='']
>>> jQuery(":text[value=]").get()
[input#jq-primarySearch]
>>> jQuery.prototype.jquery
"1.3"
>>> jQuery(":text[value='']");
Object length=1 prevObject=Object context=document
Unknown pseudo-class or pseudo-element 'text'.
[Break on this error] undefined
>>> jQuery(":text[value=]").get()
[input#jq-primarySearch]
Note that 1.3 and 1.3.2 handle it properly ( albeit with Firefox sending an error ) but they still get the node right.
Alternatively: you could use the :text[value=]
notation, which appears to work everywhere I tried it. Its just a bit suspect thats all.
( Ignore my Previous rantings, they're all bollocks, not having a good day -_-)