AttachmentSaver is Youssef Francis' extension to... save attachments... from Mail.app to your jailbroken iDevice. It's been out over a year or so, and we just pushed an update from Francis which adds iPad support. The extension runs $1.99, and works beautifully on iPad as well as iPhone.
Tap a file from Mail.app, and you'll get a popup which asks to save the file or view it. Files are saved to /var/mobile/Library/Attachments. Similar to how Safari Download Manager works from within Safari (which is also being worked on, and should be out in the next couple weeks with iPad support).
Combine this with something like iFile or AirSharing Pro (if you prefer an AppStore app), and your iPad is getting close to an actual computer.