New feature in v4.6: Search/sort by Released

The Release Date of a song / album can be displayed in iTunes, but it cannot be edited. You can look up the release date in SmartPlaylist with the detail view from the Now Playing / Selected screen:

Note: Not all mp3s have a release date, and not all release dates are properly synced to the iPhone or  iPod Touch. Songs without release date are ignored when you search by "Released".

You cannot search directly by release date, but rather by how long ago a song / album was released (similar to last played). So, when you have a rule like "Released < 1 year ago" you will always get the latest songs in your smart playlist. Easy.