Jacob has a plan. I know, Captain Obvious much?
Here's the thing: I love to flog ideas until I'm bashed in the face by the show proving them wrong. So...
Sayid's name appears on Jacob's List from the Ankh in Hurley's guitar case. From their reaction to the List, it seems the Others thought it imperative to put Sayid in the Spring. We have to think Jacob knew what would happen to the Spring after his death, as well as the time the Losties would return to the present. He knew Sayid wouldn't be put into the Spring until after his death, and, if we assume this is how Sayid became infected, that has huge implications - not the least of which is Jacob wanted Sayid to become infected.
This is all part of Jacob's plan to counter the Monster's plan. Right? Or was the Monter's plan part of Jacob's Plan all along? Could it be even the Monster - the one who knows Jacob pushes people to do things by making them think they are exercising free choice - was manipulated in this way, too?
I find these things to be more evidence for my idea Jacob is not on the side of Good or Evil, but is actually the Master Manipulator. The Monster only seems to be as powerful as Jacob because of sheer destructive force and the fact he's been around for a long time; but if Jacob gave him that power, Jacob's obviously much more powerful. And it's possible he wants Sayid to take over for the Monster now that the Monster wishes to go.
It seems Jacob may have a Xanatos Gambit going, where he'll win no matter what happens due to his having so many irons in the fire. Especially since it seems he's doing everything to protect the Island, and is obviously willing to sacrifice himself - at least in a way - to get the job done.