On my Android 4.0.4 (ICS) Xperia ray, they are stored in /mnt/asec/XXX-1/pkg.apk.
XXX is the Google Play ID of the application.
For example, Firefox is found at /mnt/asec/org.mozilla.firefox-1/pkg.apk and Skype is found at /mnt/asec/com.skype.raider-1/pkg.apk.
Following zuul's comment I took these screenshots from my phone to confirm my answer.
Firefox
Stopwatch & Countdown Timer
And these are my phone's details:
