Sorry, I really haven’t yet used the new Blender’s player feature(s). I almost always use VirtualDub to play my PNG image sequences back, it’s fast and has great visual quality imo. Plus if I need to do a quick movie, I can run out an AVI/Xvid format very fast with VDub, much faster & higher quality than Blender can do. Even when I render to a movie format from Blender (rarely), I use VDub for AVI playback & VLC Player for other formats. Never did like Blender’s player much.
EDIT: Decided to give the 2.57 player a try, discovered that there is none – you have to choose one in the User Prefs. The 2.4 player is listed, but you can link to an external viewer as well. That’s handy. Unfortunately, there may be a bug in the code that calls a chosen player, at least when trying to play an image sequence. Rather than using the first frame number of my image sequence, Blender tries to call my external player (VDub, in this case) with a filename like “MyImage???.png”, with the question marks replacing the frame number. This of course fails.