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Long press anywhere in the body of the email. Select all from the options popup, them copy.
Open iMessage, long press in the text entry space and select paste. That should out the body of the email in your text. You will have to add any attachments manually.
Well, if you just want the other person to be able to read something (not use it themselves) you could turn the iPad to portrait mode (for full screen view of the email), take a screen shot, then send that via Messages. There's a little camera icon for sending pictures.
It would look something like this.
<img src="http://www.ipadforums.net/attachment.php?attachmentid=41440"/>
If you only wanted a small part of the email, you could use Skitch to crop, add shapes or text, and even blur out sensitive portions (as I did above) before sending.