I am currently re-watching Season 5 and just got through "Jeremy Bentham". I thought the scene with Locke and Widmore at the Exit in Tunisia was very interesting. Widmore says he's "going to do everything in his power to help" John bring them back (OS). When John asks why, he says he wants to make sure John is back because there is a war coming and if he isn't on the island for it, the wrong side will win. Then they are outside, and Locke says "he said I would die". Widmore looked back in shock and said "I beg your pardon?!". Then Locke said it was Richard who said that, and Widmore says "I don't know why he said that, but I'm not going to let that happen".
If we are to believe what Widmore is saying (might be a big "if"), I think this all means he is on Jacob's side. Jacob also wanted them to come back to the island, or else he wouldn't have visited them all (especially Hurley with the guitar case). Also, if Widmore says he doesn't want John to die, he is obviously not on Nemesis's side. I just wonder how much Widmore knows about Nemesis's plot since he was so shocked that John said "he said I would die".