We did the tour of champions last year at Memphis in May. We were on the pulled pork category, and at each stop we had a Q&A session with the pit master. Every one of them injected and I asked each when they inject, and the answer was always about 2 hours before going in the smoker. None of them injected overnight. So take that for what its worth.
Between that and my own experiences and my injection recipe. I have moved to the 2 hour or even right before going in camp. Like NoBoundiers, I find if I inject overnight, what I am really doing in wet brining half the night. A lot of my pork I do a Mojo injection, and the pineapple can start to make the injection area mushy if done overnight.
The smoker is "sweating" the liquid out of the meat, so it is pulling it through the meat as it smokes and helping to distribute the flavor.