I am trying to understand how links are created to iTunes store
content. I would like them to be created dynamically, so that through
a web service I could pass certain parameters and receive a URL in
response.

So I would pass something like: 

     artist=the rolling stones&album=sticky fingers&track=bitch

And the web service would respond with a link to the iTunes store
where that paritcular item could be bought.

This link would also be part of iTunes' affilite program run through
LinkShare (www.linkshare.com).

In particular, I'm looking at a page like this:

    http://upto11.net/artistprofile.php?ar=212994

The hyperlink "iTunes" in the green "Top Songs" box provides the following URL:
    http://click.linksynergy.com/fs-bin/stat?id=8kGvM9Iw0pc&offerid=78941&type=3&subid=0&tmpid=1826&RD_PARM1=http%253A%252F%252Fphobos.apple.com%252FWebObjects%252FMZSearch.woa%252Fwa%252Fcom.apple.jingle.search.DirectAction%252FadvancedSearchResults%253FsongTerm%253DThe+Date%2526artistTerm%253DAvril%2526albumTerm%253D%2526originStoreFront%253D143441%26partnerId%3D30

This URL links to a track in the iTunes store, which is just what I
want to do. Where is the service that generates these links? How do I
access it?

According to http://www.apple.com/itunes/affiliates/faq.html

You need to be an iTunes Affiliate, and then you will have access to
the "iTunes Link Maker".

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How do I link to my favorite song/artist/album/audiobook on iTunes?

Use the iTunes Link Maker, available on the Create Links page of the
iTunes Affiliate interface. From there, search for the song, artist,
album or audiobook you?d like to link to and you will receive a URL to
add to your website and email campaigns.

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You can apply for affilation here: http://www.apple.com/storeaffiliates/