what would be the best approach to create a 'read-only' schema field in Tridion 2011? There was a good extension for 5.3/2009 (http://www.sdltridionworld.com/community/extension_overview/readonlyfields.aspx), but how should we proceed with T2011?
A GUI Extension is good way to do this in Tridion 2011. There's a useful example here:
As Nuno pointed out the example I provided is no use to you as its for Tridion 2009.
A GUI extension remains a valid approach, you just need a better example.
Once you have your code executing in the Component Edit view you can mimic the approach in the 2009 example, marking schema fields with a [READONLY] suffix.
Then in your custom JS locate the fields that have a label element that contains [READONLY].
Once you have this, the input itself will be a child element, to which you can add the disabled attribute.
That covers standard text fields, you might have to think different for other field types - but again the 2009 example will guide you!