| Okay, I haven't used Photoshop in a while, but this is possible and I could do it.
Take something like the magnetic lasso tool or magic wand tool and select just the flowers (or whatever it is you want to have in color). Then go Select -> Invert or something along those lines, to invert the selection (so everything BUT the flowers is selected). You could feather the selection a bit at this point too, it might make it more realistic, but you can always just experiment and see if it works that way. Anyway, after that you simply go Image -> Adjust -> Hue/saturation/lightness or something like that and drag saturation all the way down to 0. Deselect and you're done (alternatively after selecting and inverting the selection you can just hit the Desaturate tool, which if I recall correctly is also in the Image menu somewhere).
Note that the above will make the rest of the image grayscale, not completely black and white. If you want it completely black and white, you can then go to image -> Adjust -> Brightness/contrast and set the contrast up to max. Note that this won't end up looking very good.
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