I came across a booklet made by SIPA that contained admit rates for what they deemed were "peer schools" (I think the booklet was for SIPA admin eyes only, though). While these numbers are a bit old, I think they give a rough idea.
SIPA (MPA) [2008]: 51% (336/661)
SIPA (MPA) [2007]: 58% (337/578)
SIPA (MIA) [2008]: 40% (732/1,815)
SIPA (MIA) [2007]: 49% (804/1,626)
HKS (MPP) [2007]: 26.9% (340/1,262)
HKS (MPP) [2006]: 28.7% (386/1,344)
WWS (MPA) [2008]: 15% (116/765)
WWS (MPA) [2007]: 13% (103/792)
Maxwell (MPA) [2008]: 67% (243/361)
Maxwell (MPA) [2007]: 71% (281/394)
SFS (MSFS) [2008]: 29% (268/922)
SFS (MSFS) [2007]: 26.4% (220/834)
SAIS (MAIA) [2008]: 52% (791/1,521)
SAIS (MAIA) [2006]: 39% (640/1,639)
Fletcher (MALD) [2008]: 43% (699/1,620)
Fletcher (MALD) [2007]: 34.1% (558/1,636)