How (or is) "low & slow" turkey safe?
This article, by a reputable food scientist, summarizes the possible dangers inherent in slow cooking of turkeys, with some scientific citations and actual experimental data on microbiological growth in slow-cooked turkeys.  I'd encourage anyone interested in slow cooking to read it to appreciate the great variety of microbes which could cause problems, as well as the problems that can occur with persistent toxins or spores.

Basically, the take-home message seems to be if your turkey gets to at least 130F within 8 hours and ultimately reaches 165F throughout, it should ultimately be safe, according to the cited research.  However, this is of course at odds with official FDA and USDA standards which seemingly recommend less time in the "danger zone" (though they seem to allow some exceptions for food that is smoked or cooked in a pit barbecue in their guidelines, which they sometimes allow to take up to 8-12 hours to attain temperature, and the USDA's own turkey cooking guidelines clearly imply that large turkeys will spend longer than a couple hours in the "danger zone" when roasted at their "safe" minimum temperature of 325F, though other users here have asked them about this, and they refuse to acknowledge that fact when queried).

I explored related microbial issues of slow cooking in my answer to another question, drawing in part on the article I cited earlier.  In sum, the official "danger zone" guidelines have lots of built-in buffers to allow for other errors in food handling.  Some microbiologists are on record as saying that these guidelines are overly cautious, but exactly how far you can "bend the rules" is not something I want to speculate on.  

Obviously, the safest course would be to follow official approved guidelines.  I personally had done a lot of research into this before I ever slow-cooked anything myself, and I would encourage others to do the same before coming to their own conclusions.  One important thing I would note, however, is that there are limits to slow-cooking safety -- for example, I absolutely would NOT eat poultry that, say, spent a day or more in the danger zone before reaching 130F.  Toxins will build up eventually, and at some point even boiling the turkey won't be enough to make it safe to eat anymore.