As leaked earlier, Wishlists have reached the Google Play Store. We just noticed the feature on our Galaxy Nexus and it seems to be a part of already available Google Play v 3.9.16. So, there is no need to download any update to Google Play client app, it will simple appear on your phone as Google enables it for your account.
A new Add to Wishlist icon now also appears near the app or any other content title and you can use that to add that particular app to your Wishlist.
You can track you Wishlist by going to menu>>My Wishlist.
Google has been silently bringing many feature to Google Play in the last few months and we hope to see more in Android 4.2, which is set to launch on October 29, as per the invite sent out by Google earlier today