I've just returned from a conference in France, having taken my new phone with me. Since the powered-on phone lasts about a day before it needs recharging, and since UK and French power sockets are incompatible, I needed a mains plug adaptor. I took two, and they both worked fine.
I took the cheap and cheerful "Tourist Mains Travel Adaptor". The reviews are contradictory about whether it works in France: it does. The product shown in the advert photo wouldn't, mind you, as it has no hole for the French earth pin (below left). But the one delivered to me does (below right). It's small, light, and cheap.
Because I wasn't sure if that one would work, I also took a "Swiss Travel Products World Travel Adapter": more expensive, and definitely snazzier. This does the full (UK or EU or US or Australia) to (UK or EU or US or Australia) conversion: it has four sets of sockets on one side, and buttons to extrude one of four sets of plugs on the other. Since all the plug pins can be retracted (below, left), it makes it convenient to carry. I tested the UK plug extrusion (at home), and the EU plug (below, right) in France. The slimline plug has no need for a hole to take the French earth pin.
Verdict: both work. The smaller cheaper one is fine for France. The larger, more expensive one means you need only the one device for all your foreign power needs.
So I had a fully charged phone all week, and could neurotically check my email several times a day.
UPDATE (27 Aug 2011): they both work in Norway, too.