The OS X version on the old disk usually needs to be newer than the hardware you're putting it in. This is because when Apple releases new hardware they usually put a newer build on it with the required hardware support. As soon as they release the next decimal upgrade you should be fine to swap installs though.
IOW if you just bought a unibody MBP and your MB disk has 10.5.5 chances are it won't work until Apple releases 10.5.6 and you install that on the MB. If your MBP is a refurb unit and you have 10.5.5 on your MB disk you should be fine though.
You could of course install the disk with the older OS in the unibody MBP anyway. It won't boot from that disk, but you can boot from the install DVD and do an upgrade install. That way you can retain all your apps, documents, setting, etc. And finally then there's still the option of suing MA to migrate all your MB stuff to your new MBP.
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