It's using 4.0 APIs, according to Matt Seigler's story and interview with Google SVP of Chrome Sundar Pichai:
"Back to the bad news: some of the more advanced features of Chrome for Android require APIs found only in Ice Cream Sandwich, so the team made the call to make it only available for Android 4.0 and beyond. Again, this means only 1% of current Android users out there can actually get and use the browser right now."
- http://parislemon.com/post/17215781807/chrome-for-android-th...
1% of Android users is still at least a couple million. That's more than enough to get a reasonable pool of beta testers. By the time it gets out of beta, the Android release distribution will probably look quite different.
"Back to the bad news: some of the more advanced features of Chrome for Android require APIs found only in Ice Cream Sandwich, so the team made the call to make it only available for Android 4.0 and beyond. Again, this means only 1% of current Android users out there can actually get and use the browser right now." - http://parislemon.com/post/17215781807/chrome-for-android-th...