If you are after very long battery life,
get one of the Sony S740 series. This player is not yet available in the US, but oddly enough is available in Canada, Europe and Australia, and is rated for 45 hours of battery life. It's a better option since you are not fussed about the speakers on the S540 series, so maybe you can look at import options.
The A series is being replaced by a new A840 series, which is again not yet available in US but is now available in Europ eand Australia. It has the digital amplifier of the X series, which is pretty widely praised on this forum. Perhaps this is why it is hard to find the A series at the moment.
The walkman's will work drag and drop on the Mac, but for full integration with iTunes I use a neat little program called Salling Media Sync. It costs a little money but it handles podcasts and music just fine, and synchronises with playlists so you can work with smart playlists, ratings etc. I find it much more useful than the drag and drop, because I found (on my S series walkman) that the thing didn't want to eject properly.
I don't know what the above poster is referencing, because the S series can certainly do folder searching, but does not have a mini USB slot :S