I'm having the same problem as well, but I'm using .Mac, not GMail.

I've got messages that I KNOW have images attached.

The N900's email app shows an attachment icon in my list of emails.

When I open the message, there is no sign whatsoever of the attachment: nothing in the menu bar, no links to the attachment files, no option to download anything.

As others have said, looking at Message Details shows message sizes that are consistent with the attachments being somewhere, but I can't access them at all.

It's incredibly frustrating.