Google’s Product search is lumped into organic referrals in Google Analytics if you don’t use tracking parameters, which can distort the level of true natural search volume you see in any web analytics package.
A typical solution shown on some websites is to add ‘utm_’ tracking variables such as http://example.com/?utm_source=google&utm_medium=products to the URL, which correctly identifies the traffic as being from Google Product search (whether it’s the actual shopping search or a Google onebox). However this solution sacrifices the referring keyword data, and just shows the site as a referring website.
The simple solution to this is to change the utm_medium parameter to ‘organic’ (and the utm_source to ‘googlebase’ or similar), which Google Analytics then treats as a normal organic search engine, and accordingly reports the referring keyword 
Update: Tripleox correctly pointed out on twitter that you can still get keyword data with the first method by going to Traffic Sources -> google / products and use the dimensions drop-down menu to select keywords.
