If you look at the "Recently Added" playlist (on the iPod) through iTunes, does it show any tracks in there?
Playlists with no content won't show up on the iPod, so in the case of a smart playlist, if the rule-set eliminates all possible tracks, then the playlist will appear to not be there (rather than simply appearing as an empty playlist).
Also make sure you check the date and time on your iPod, since this playlist is based on date/time, so if that's wrong, then you may not have any tracks that would meet the smart playlist criteria.
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