Stumbled across these apps the other day... not terribly useful for me, but I'm sure some can use them. Basically, when you hit the "Preview" button for an attachment you have in Gmail (or the "Open" button in generic Email), you can choose one of these apps, and it'll let you save it to the SD card.
Save my Attach - Android app on AppBrain (Free)
Blackmoon AttachSave Lite - Android app on AppBrain (Free)
Blackmoon AttachSave - Android app on AppBrain (Paid, supposedly better at preserving the attachment filename than the free apps)
Attachment to SD - Android app on AppBrain (Free)
Save my Attach - Android app on AppBrain (Free)
Blackmoon AttachSave Lite - Android app on AppBrain (Free)
Blackmoon AttachSave - Android app on AppBrain (Paid, supposedly better at preserving the attachment filename than the free apps)
Attachment to SD - Android app on AppBrain (Free)