Skype team might not have added support for Samsung Galaxy S or S II in the updated release of Skype App for video calls, but theos0o at XDA has already done it. He has patched the Skype app to make it work with both the devices and you can now easily make video calls from your Samsung Galaxy S or S II using the app.
You can grab the patched app download from here. This will only work if you SGS has Android 2.3 update installed, and SGS2 comes pre-installed with it so no worries.
Thanks Rushabh for the tip