I agree with Todd
Neal (San Jose)
>Sharpen the blade and take it for a spin. You don't say what kind of plane it is, but if it has a frog, small cracks behind the mouth might not make any difference. I have an Ohio Tool 4 1/2 with cracks just as you describe. Except for having to flatten the sole(the cracked part was also bent down a tad), fettling this plane into a good user was not a big deal. I use it as a "rough" smoother.
I suspect the cracks in my plane were caused either by someone that didn't know what he was doing got the frog back too far, which put the blade in contact with the sole, then tightened something way too tight. Another possibility is that a previous user had the frog back to open up the mouth for some rough work, and hit a knot, driving the blade back.
If this is a block plane, then it isn't as good a bet, as the sole provides some support for the blade. But, whatcha got to lose?
Neal