Right now the top result when searching for changing chrome user agent string is terrifyingly bad and advocates editing chrome.dll with a hex editor. Don't do that, maybe it was required in the early days, but current builds support a --user-agent paramater. You can just run:
$ /path/to/chrome --user-agent="Googlebot/2.1 (+http://www.google.com/bot.html)"
... and impersonate any user agent you can think of. I mean yeah, running my browser from the command line isn't something I do a lot either, but it's a whole lot easier, and less likely to fail spectacularly, than editing a shared library with a hex-editor.
Masquerading as the iPhone
I set up a short cut (yes, I do use Windows at work), that put Chrome into pretending-to-be-the-iPhone-mode and gave it the yellow Chrome icon so that I can easily start up an appropriately configured Chrome session (Still easier than modifying a DLL with a hex-editor):
chrome.exe --user-agent="Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3"
... look-at-me, I'm the iPhone. Yay!