How can I print a PDF X-1A from a iWork Pages document?
If they can take PDF-X1/a, then they can take any of the others as well; you shouldn't have any problems. 

PDF-X1/a was simply the first in a long line of iterations of a new PDF schema that basically does a lot of preflighting for you (it's one of the best things to happen in the printing industry in a long time). It means your document is "press ready", which mainly applies to SWOP presses but can apply to small printers as well.

As a footnote to all of that...   You can also output press-ready PDFs using the Mac OS X-supplied "Save as PDF" button in the print dialog box from just about any application.