Found UI guidelines of Launcher icon design http://d.android.com/guide/practices/ui_guidelines/icon_design.html but System Icons in recent Android releases doesn't seem to follow these guidelines (icons are flat instead simplified 3D perspective, as described in document).
Are they already obsolete and what are the current guidelines ?
They don't seem to have updated the guide since the Android 1.5 release, and Android 2.0/2.1 do things very differently for the Launcher icons. I've been effectively ignoring the guide when it comes to icons ever since the Android 2.0 release came out, though most of the other information in the guide seems to apply still (notifications, menu icons, etc.). Since no one enforces the guidelines anyways, it's better to make your app fit with the recent and future versions of Android than to follow the three-major-releases-old outdated guide.
They haven't released any specs or templates for the new flat icons, but the icons for most of the system apps are available in the Android source code to use as reference if you do a checkout (packages/apps/(appname)/res/drawable-hdpi/app_(appname).png, for most of them).
Launcher icon guidelines (according to a tweet by Romain Guy) 72x72, flat and no perspective.
It's worth noting that while the menu and notification icons appear the same, tab icons have also changed. The tab icon style is the same, but the colour scheme is different, using mid-grey icons for inactive tabs, and white icons for foreground tabs.
The launcher icon guidelines have just been updated for Eclair: