I've programmed a LOT of 18F2331's, all of them with MELabs programmers (I have 3 versions; epic, serial and USB). Here is what I've found:

For ICSP I put in a jumper to disconnect the programming power supply from the rest of the board. That way only the PIC is powered when the programmer is in use. When I unplug the cable, the jumper is put on and the pic gets power from the board.

Today I was programming some DIPs repeatedly, using the MELabs programmer and a zif adaptor. I had to add a jumper to the adaptor, as shown on their web site. (I think my adaptor was almost 10 years old!) Because the chip was being used on a development board, I didn't have the luxury of putting on a jumper. I was not surprised to find the chip wouldn't program ICSP.

A couple months ago I did run in to a couple bad 18F2331 PICs. Some new code worked perfectly on several chips, but on several it did not. I was thankful that I didn't get the faulty chips for the first one, or I would have suspected my code!