Same results here. The Sign-in function on the main WhereIGo page has some javascript in it that the iPhone does not like. I've tried a few work-arounds, but can't get the cookie to set itself so I can download a cartridge.
I checked the PiGo site, and sure enough, it is a known issue. You can still download the cartridges through iTunes though.
From the site:
1) Sometimes logging into the web site doesn't work! Groundspeak is looking into this. Meanwhile, we've tested a new login button and will add that in the next update. Also, with newer devices, you can download cartridges with your computer and drag and drop to the PiGo app folder in iTunes.
2) Be sure to select "PocketPC" as the device when downloading cartridges for best results, although any device should work.