Do any iPad e-book readers support sharing excerpts?
Amazon supports a "public notes" feature.  When you create a note or a highlight on a section using a Kindle or any of their apps, including the iPad app, it's uploaded to the Amazon servers and visible to you at https://kindle.amazon.com/kindle.   From there you can copy and paste into Instapaper or email if you wish.

On that site, you can make your notes public so that other people can see them.

For more information see https://kindle.amazon.com/faq