Mitsubishi LT-46131 and LT-46231 offer the best PIP options of any 46"+1080p LCD on the market (even with 1080p sources); google those model numbers for Mitsubishi's website so you can download the manual. The Samsung 4695D, 4696, 5296D and 57" model 1080p LCD also have PIP options, although much more limited than the Mitsu models. The JVC (LT-46FN97 and LT-46FH97), Toshiba (47HL/LX/LZ96) and Westinghouse (47" 1080p monitor) 1080p LCD's also offer some form of PIP but with limitations (i.e. limited number of inputs can be seen simultaneously).
Westinghouse 37W3 also has PIP. As mentioned above there are some limitations: there are 2 processing boards inside, I think the inputs have to be coming from 2 different boards to show at the same time. In other words if HDMI and DVI2 are both processed by board number 1, they can't be displayed in PIP mode at the same, but you can use each one of them with DVI1 input which is processed by board #2.