You're right.. reverse engineering the data format isn't illegal, using their data would be.. Knowing what's in the data isn't really useful though, if you can't use the data you figure out.

Looking at the data in the Pocket Streets downloadable map files, I wouldn't even attempt to reverse engineer the files. The stuff from mapopolis are easy to figure out.. looks like[coordinates, other info, street name] chunks, where the Pocket Streets info is stored much differently..