(Update September 2012):
MRT points out in the comments that “Eclipse Version” question references a .eclipseproduct in the main folder, and it contains:
name=Eclipse Platform
id=org.eclipse.platform
version=3.x.0
So that seems more straightforward than my original answer below.
Also, Neeme Praks mentions below that there is a eclipse/configuration/config.ini which includes a line like:
eclipse.buildId=4.4.1.M20140925-0400
Again easier to find, as those are Java properties set and found with System.getProperty("eclipse.buildId").
Original answer (April 2009)
For Eclipse Helios 3.6, you can deduce the Eclipse Platform version directly from the About screen:
It is a combination of the Eclipse global version and the build Id:
Here is an example for Eclipse 3.6M6:
The version would be: 3.6.0.v201003121448, after the version 3.6.0 and the build Id I20100312-1448 (an Integration build from March 12th, 2010 at 14h48
To see it more easily, click on “Plugin Details” and sort by Version.
Note: Eclipse3.6 has a brand new cool logo:
And you can see the build Id now being displayed during the loading step of the different plugin.