Using a Mac keyboard on a PC: try the "driver" at the top-right of this page:
http://matias.ca/tactilepro3/support/index.php; for some reason it's missing from the support page for the Tactile Pro 4. This is a compiled AutoHotkey script that swaps the keys. May not work in XP (completely useless for me when I tried — the alt key would constantly jam during alt+tab) but works fine in Windows 7, likely due to limitations in XP. For some reason Matias forgot to add an icon to the EXE or hide the tray icon; this is really easy to do if you want to build a custom version.
Mac OS X has native UI for remapping the modifier keys in System Preferences, so using a PC keyboard on a Mac works just fine.
Obviously in both arrangements there will be keys in the wrong places, keys that do nothing, and functionality that you'd like but don't have the key for. For example, there's no pause/break key functionality when using an Apple "Aluminium" Keyboard on a Windows computer. You could probably remap that in using AutoHotkey.