The only reason you would need the iPad baseband is if you need a 3G or 3GS unlocked and it's on either the 5.14.02 or 5.15.04 baseband-- since these basebands cannot be unlocked via ultrasn0w. Since you *can't* downgrade basebands, the only option is to upgrade to the iPad baseband 6.15.00 which is unlockable.
The only catch here is that if you have a phone with the iPad baseband installed, it's hard to install/reinstall a new firmware (Read Rich's explanation above). On a 3G, you can get around this by installing an older firmware (I typically use 4.1) and it should go through just fine. On a 3GS, you typically don't have this luxury since Apple forces you to update to the latest firmware only (unless you have your SHSH blobs backed up... but that's a whole other story).
If you find yourself in a situation where you have an iPhone with the iPad baseband installed and need to install a firmware that rejects the iPad's baseband, the solution is to make custom firmware (via sn0wBreeze) that preserves the baseband. Being that these custom firmware's don't touch the baseband, they will install just fine.
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