Unfortunately there is no way to search within the site to see who has a certain book on their wishlists.  However, one very clever user has found this method using Google:

In the Google search box, type in the book title, then put a space, then "site:http://www.readitswapit.co.uk/UserWishlist.aspx" (without the quote marks) and click search. You will then see a list of all users who have that particular title on their wishlists. 

Two Points that people have noticed:

1) Sometimes it will show up lists that when you click to look at them, don't have the book listed, and some users may not be displayed even though they have the book on their wishlist.  This is due to the caching intervals at Google - it only takes a snapshot of data on the site every so often, so will miss anything posted in between caches. 

2) Sometimes if you scroll to the bottom of the google searches it will say:

In order to show you the most relevant results, we have omitted some entries very similar to the 1 already displayed.
If you like, you can repeat the search with the omitted results included.

Click the "repeat search with omitted results included" link and you will see more results.

Thats great Penny. I'll save this to my favourite threads and then use it when necessary.  It's annoying when you know someone has asked for a book and you forget who it is, so this will come in really useful. Thank you

Foreternity (the genius who discovered this!) had posted this elsewhere, but while tidying the forums up today I thought it would be a good idea to have it stickied here in the Info section. 

Great advice, just used it but couldn't find a way to contact the user who I can swap a book with! Just started! Thanks.

Very useful, thanks!