How to set default apps on Samsung Galaxy S II + Android 4.0 (ICS) On stock Android and 2.x ROMs on SGS2, default apps could be set in Settings -> Applications. However, when I go there with the ICS ROM, I don't see where I can set which default applications for certain actions.

Where are these settings found in SGS2 with Android 4.0?
Go to SETTINGS > MANAGE APPLICATIONS or APPS (depending on your ROM and version). Scroll through the list of apps and tap on an app that is the default for some action. This will bring up the app's properties that show the size of the app and other relevant stuff.

If the app is set as the default for a particular action - the CLEAR DEFAULT button will be active, else it'll be grayed out.

As an alternative - check out Default App Manager - it's an app that has similar functionality.