Determine when OS X was installed?
You can use the pkgutil command to get info on the packages installed on your Mac; pkgutil --pkg-info com.apple.pkg.BaseSystem should list the installation time for the base system. But it'll be in the form of seconds since the unix epoch, so you need to use date -r to convert that into a human-readable format. Here's an example:

$ pkgutil --pkg-info com.apple.pkg.BaseSystem
package-id: com.apple.pkg.BaseSystem
version: 10.6.0.1.1.1249367152
volume: /
location: /
install-time: 1306707387
groups: com.apple.snowleopard-repair-permissions.pkg-group com.apple.FindSystemFiles.pkg-group
$ date -r 1306707387
Sun May 29 15:16:27 PDT 2011


EDIT: I misunderstood the question a bit -- I didn't realize the OS had been updated since 10.6. Unfortunately, the receipts system doesn't generally records for more than one version of a specific installer package (as defined by the package ID), so if you've installed more than one version of "com.apple.pkg.BaseSystem" it'll only have records from one of them. But you can find the oldest package you have records of, and there's a good chance it's a holdover from the initial system installation:

pkgutil --pkgs | while read pkgname; do
    installtime="$(pkgutil --pkg-info "$pkgname" | awk '/^install-time:/ { print $2 }')"
    echo "$installtime $pkgname"
done | sort -g | head


(Then use date -r to convert to the timestamp to a readable form, as before.)