It is possible to kill the city bosses!
We killed the Silvermoon boss last night, 40 people, no horde was around.
http://forums.wow-europe.com/thread.html;jsessionid=C10128EF5F0CCB60A814376961B706C2.app06_06?topicId=6365446677&sid=1
We also tryed trall in org, however having alot more guards then any other, it was a short try, and we got overwhelmed by atleast 25-35 hordeshaving fun.
The undercity boss evaded many times for us, and finally ended up killing the raid, it seems she bugged at the stairs.
Its true they are level 80, and we did only hit them for about 500-1200 (warlock normally hitting for 3-4k noncrit), and many times you just miss. It seemed that the melee had a bigger chance of hitting him, very fast they were all top dps on recount.
It took about 15 minutes to pull 5mil hp off the boss, we had easy guards respawning every 2-3 minutes, and the rather "hard" guards respawn every 5-10 minutes.