You use the same technique as Forwarding a text to a phone number, but enter an e-mail address in the "To" window instead of a phone number:
Assuming you're using the stock Texting client...
- Long-press on the message and a window should pop up, and one of the options will be Forward Message.
- Select Forward Message.
- In the To: box, type in an e-mail address.
It seems that if you just enter a name in the "To" box it will only give you the option of the phone number associated with that name, but it will accept an e-mail address if you type it in manually. I just-now tried it and forwarded a text message to my e-mail address.