Epson's new PrintJinni app will let you beam email-attached Microsoft Office files, PDFs and JPGs to your printer, without the need for an extra computer. Epson claims that WiFi transfer process will maintain full document layout integrity, for those concerned. [iTunes]
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Or wait until iOS 4.2 and it'll be built in. Good for older units if it doesn't require 4. Steep price though. $7 intro price.