Can any apps print from the iPhone?
If you want to print pictures and photos, you can use Epson iPrint (there are similar apps from other printer manufacturers, e.g. Canon iEPP, HP iPrint Photo 2.0, Kodak Pic Flick or Lexmark LexPrint). For printing PDFs or other documents, the already mentioned "print n share", "Print Bureau" (both from eurosmartz) or Air Sharing Pro from Avatron could be useful.