Some of the forementioned examples are pretty shocking.
However, this is nothing when you consider American actors attempting British ones. They can only do one accent for starters (London) and even this is a pretty awful representation. For reference, see Frasier, where Daphne's (she's from Northern England, and an actual British actor) brother comes into it and has this bloody awful cockney accent...
On an overall point, these actors are paid millions for their skills, yet only there are relatively few are competent enough to accurately portray various accents. Proves that many are chosen purely on looks alone. War films are the best example, if playing a German/Russian etc:
1. Speak the language/get actor from country and use subtitles. Obviously will never happen in Hollywood due to the fact that the average US citizen can't cope with this concept
2. Put on a good accent
3. Never, ever take the role if you can't do any of the above.
Example - Enemy at the Gates. Great film, but Jude Law should have had a Russian accent, Ed Harris should have had a German one. It was so false without this, and spoilt it IMHO.