A poor workaround, but you can forward the email, do your selecting and copying on the edit screen, then carefully cancel without sending, then head to whatever app you wanted to do your pasting in.
There is the Copy Paste It app in the Market that offers universal copy&paste, but it is OCR based. It requires rooting the phone. Reviews say that it isn't always perfectly accurate. Mixed reviews. It is probably good enough for general use, where accuracy isn't a 100% requirement.
I haven't tried it, mostly because I'm still reluctant to root.
When you are viewing a message in Gmail, press the menu key, then choose more and then Select Text. Then you simply highlight the text you want with your finger and it is automatically copied. It is kinda tough to do accurately though, so I usually select more than I want then when I paste it I delete what I don't want.