MDLP was refered to in the minidisc node, but I had no idea it is, so I decided to so some research to figure it out -- this is what I've come up with. A large part of my information has come from the MDLP FAQ at http://www.minidisc.org/mdlpfaq.html and the official Sony MDLP site at http://www.world.sony.com/Electronics/ATRAC3/index.html.
MDLP Stands for MiniDisc Long Play, and it's exactly what the name implies. Depending on which of the two modes that are used, you either get 160 Minutes or 320 minutes of audio on an 80 minute MD. The difference between the formats is LP2 is distinct stereo with each channel being encoded as it's own stream and LP4, where only the differences between each channel are encoded (aka Joint Stereo).
MDLP is just another name for ATRAC 3 on MD. Version 3 differs from the older versions in that it is frame based, not waveform based as in ATRAC 1. ATRAC 2 was adapted into ATRAC 3, so that's why you never hear about it.
Of course, you need a player that can handle MDLP/Atrac 3 to actually hear the audio -- if you play an MDLP Track on a non-MDLP Player, you'll get nothing but silence. This is also another problem. The FAQ says it better than I: "In LP2 and LP4 modes 20 bytes of dummy data per 212 byte soundgroup are used to keep compatibility with SP mode, making them appear as normal ATRAC soundgroups containing silence... You mean that 10% of an MD's capacity is wasted in LP mode due to lack of planning? Yes.
It's bitrate appears to be on par with an average-quality Mp3, with LP2 checking in at about 132kbps and LP4 is 66kbps. LP4 is lower due to, again, the use of 'joint stereo'.
From what I understand, MDLP LP2 is very close in audio quality and artifiacting to CD, while LP4 sounds much like Atrac 1 v4.