I think that this should be a feature available to all users. For example, an admin creates the board at home, but when he/she gets to work, it wont allow login to admin cp. But turning this off would allow them to login no matter whey they are. I think that it should be optional for situations just like this.

OR

Incase someone only uses their homecomputer & wants added security, they can turn it on if they are afraid of someone hacking their account or whatever.

To disable IP checking for the ACP, open up admin.php:

Find:
$check_ip = 1;

Replace with:
$check_ip = 0;