I noticed on someone wish list that I had a book that they wanted but mine was paperback instead of hardback. Is it improper to pm  them and let know that you have one in a paperback version?

I'm clueless with ettiquette so I don't know about PMing, but you could always try the****May Wishlisted Books with Different ISBNs****  thread in the Book Bazaar.  Those sorts of threads (i.e. Books you have the only copy of, Books you're looking for) seem to get good results. 

How many of your paperback version are available? You can find this out by putting the book on your reminder list.  If there's other also posted-chances are they already know it's available in paperback and they really want the hardback version.  If yours is the only one, you could tag the hardback book w/"posted in paperback w/ISBN#xxx) for them to see when they look at their WL.  My guess is they want the hardback version only.

James' suggestion is a good one.  In general, when an alternate version of a wishlisted book is posted, an e-mail goes out to those who have that title (different ISBN) wishlisted.  (Some members have turned off this option in their preferences.)

I'm assuming that you were browsing someone's wishlist when you saw it.  If you have a relationship with the person who has it on their bookshelf -- you've traded PMs or books with him, or "know" her from a forum discussion -- a PM wouldn't be out of line at all. 

some people want hardback books for their keeper shelf and are not interested in paperback. i know i am only interested in hardback copies of the harry potter books.  there are tons of paperback copies, but i am not interested in them

I've found that the feature that's supposed to let us know if there are other editions available is unreliable.   Many of us will WL all available editions, but still, if it were my wishlist, I would LOVE it if you PM'd me to give me that option!

I think a pm would be fine.

I don't see why you couldn't PM them. The worst they'll do is say they don't want it, right?

I've sent the following message to Bookmoochers in the past with a subject heading "A book on your wishlist"

"Hi - I noticed you have 'The Life and Times of Joe Schmoe' requested under two ISBN's. I have a third ISBN (British paperback, for example) 978-XXXXXXXXXX on my TBR pile. If you're interested, you might want to add that number as well so you're notified when I post it to the system."

Most folks have thanked me, and all but one have added my ISBN as well.

Here at PBS (where there isn't a public list of wishers), in your case I'd have no hesitation in sending a similar message that I'm ready to post a different binding of a requested book.

Ok, why don't I see it?  

 In general, when an alternate version of a wishlisted book is posted, an e-mail goes out to those who have that title (different ISBN) wishlisted.  (Some members have turned off this option in their preferences.)

Where is this option?

Go to "my account" then "settings" and click on e-mail--there you can choose what you want to be notified of via e-mail (or not) and the one you want is "Wishlist other versions."

Cheryl