This may be a good summary for you on this topic:
http://en.wikipedia.org/wiki/SolderSome of the big things that we are going to relive in my opinion with lead-free is "Tin Wiskering". There are a few companies that have taken up some strong testing iniatives to test for this, but we all know alot wont. Since this phenomenon is not fully understood one thing we can almost see to happen again is premature failure of items due to short circuiting. Either way, check the link above, and the above statements is just my rant.
Hope this was helpful!
Forum