After 5 years of asking that I found the answer...GRILL them!! I cooked many a loin and had very good results, but I now grill them and my results are much better than smoking.
I really do believe there are certain meats that simply don't lend themselves to low and slow cooking...pork loin is one of them. Don't get me wrong, it can be done. It's just so much easier and more predictable to do over high heat.
So, if you insist on smoking, I think you should smoke over higher heat....similar to chicken....300-325�. The internal temp should be NO higher than 145�. Then let it rest for 5-15 minutes before serving and of course slice thin.
For my grilling techniques check out the link....look for the article entitled "The Secret to Grilling Great Pork".
http://www.recipegoldmine.com/bbqguru/bbqguru.html Hope this helps and hope I don't offend anyone by taking the "grilling route"!!