Phil - If you could post the link to a working example that would be great! Running on background or popup both fail for me as the url is coming back with chrome-extension://..... instead of na35.salesforce.com...

Is your code being ran from a background script?

After a lot of examination ForceTK does NOT work OOTB for chrome extensions. It's looking for hostnames and protocols (which are not typical from a chrome extension) attempts to proxy based on those variables.

What does this mean? ForceTK requires that you add the correct REST endpoint hostname for your instance (i.e. na1.salesforce.com or similar) as a remote site in Your Name > Administration Setup > Security Controls > Remote Site Settings.

When I run this with the current sessionID I'm getting this; Failed to load resource: net::ERR_FILE_NOT_FOUND chrome-extension://okgglkjjoapejcmbmegeclmnfmdamool/services‌​/data/v29.0/query?q=‌​SELECT%20Id%2C%20Nam‌​e%20FROM%20Opportuni‌​ty&_=1474424357089. Looks like forcetk is not being passed the instance id. How do I do that?

Yes, chrome has a cookie api. Thx!