For example, if I wanted to create a note about gifts to purchase for others, and then had a note for one of the people on those list, could I link the notes between each other?
There's a hack to doing it on the Mac by using the exploit of spotlight, however this will only work on the same Mac. As well as linking to notes using the web client, however this will force the notes to be opened up in a browser.
This feature was introduced by Evernote for Windows and Mac users in June 2011. More information in this Evernote blog post.
This feature is available in Evernote for Mac 2.2 Beta 1, and Evernote for Windos 4.4 Prerelease (and, assumably, in future standard releases).
"Copy Note Link" appears under the Edit menu. It is also available in the right-click menu on notes. This link can be pasted into Evernote or another application.
If pasted in a plain text document, you can see the link is of the format: evernote:///view/...