In case anyone else is banging their head against this:
Some apps now have icons called icon@2x~ipad.png. Some of those icons are at 148x152 and some aren't. Some have icons called Icon-72.png, some Iconemail@example.com, some Icon@2x.png and some use a lower-case i and some don't and some don't care. Some have none of those but have icons called specific names. I guess you can go and look in /Applications/<appname> to find out exactly.
On a Mac you can use pngcrush to 'un-optimize' the native iPad/iPhone PNGs (if you have the iPhone dev kit installed) and you can use sips to resize a PNG (the advantage for me over using a GUI is that I could write a script to resize and rename a bunch of icons at the same time).
This is how to un-optimize a PNG:
This is a script that resize all PNGs with @2x in the name in the current directory and all its subdirectories and to 148x152 and appends ~ipad.png to the new file name:
/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/pngcrush -revert-iphone-optimizations -q <input file> <output file>
Apologies if I'm repeating stuff that everyone already knows, but I can't find this stuff anywhere.
for i in `find . -name "*@2x*.png"`
dir=`dirname $i | sed -e s%./%%`
outfile=$dir/`basename $i .png`~ipad.png
sips -z 152 148 $i --out $outfile