This is something that comes up often on the Google Analytics Help Forum
Unfortunately the Tumblr help docs for installing Google Analytics are showing a somewhat very out of date version of tracking code (note the image that appears to show part of the 'old' urchin.js - meaning the help doc hasn't been updated for a while!) ... and Tumblr.com for some reason doesn't seem to like the current asynchronous version of the GA code, which got me thinking on how to go about it ...
At this point in time, my suggested method seems to work without problem
