I guess I have to disagree with the "banning" statement - while Google does not like dupe content, the worst that should happen is they will pick one page as the original page and give it the PR (for all that it matters anymore) and the rest just dont get any credit - the reason I would say that they wouldnt go any further than that is: think about all of the non-adult hosts out there - Angelfire etc. where many people do host - and do get loads of sites listed in Google. If I wanted to make you possibly get banned all I would have to do is copy your website and put it up on my hosting account. Since it would be impossible to account for this in a mathematical algorithm, unless you are out there generating thousands of doorways and mirrors that would raise a red flag and a "human" look-see, I would say that you're pretty safe using mirrors.
This does not take into account any specific LL rules or their wishes - I just think that people tend to overlook other reasons for problems with pages and automatically assume that bans have been placed.
I've seen quite a few actual bans and in every case - someone was scamming or doing something that put you way above Googles (or your competitors) radar. WMs have been building free sites for years, and have been able to get one copy listed in DMOZ and had that with the most important recips on it - while submitting several mirrors, yet I still see them placing quite well for their search phrases after 3 or 4 years.
I guess overall I just have this tick that goes off in the back of my head every time I hear people worrying about getting banned as it so rarely really happens - a few penalties get thrown around without anything to base them on other than a guess, and the ones of us that have had sites banned or penalized usually know exactly what we did wrong
- The penalty to the mirrors is something I can live with if I get the niche traffic to those mirrors, and the primary page (usually my index.html) gets good SE traffic.