In the GIMPS database we can only see if an exponent is "Factored", but it can be difficult to find out wether it has been completely factored or not. On my wishlist is a status called "Completely factored" for those exponents which are proved to be completely factored.
You can find the completely factored Mersenne expos here:
http://www.garlic.com/~wedgingt/factoredM.txt
So, if I understand it rightly M1061 (now M1277) was previously the smallest expo without know factors, but M929 is yet the smallest exponent which is not completely factored.