Snow Leopard’s license did not allow for unlimited installs. One machine. That’s it. It has always been
possible (not
legal, though) to install Snow Leopard on as many machines as you wanted to. (That has been the case for every version of OS X since the beginning.) It has also always been possible to install it without even owning Leopard – even though the license didn’t allow you to. You could buy the family pack for a few dollars more but that also wasn’t unrestricted. It allowed for five users.
As for Lion, you can just log in with whatever account you want in the Mac App Store. That’s it. It’s not a problem. Logging out won’t disable any apps. You can download Lion as often as you want. (If you don’t want to use the Mac App Store you can just extract the installer DMG from the Lion app and use it to install Lion on all machines you own. This also saves you from having to download Lion twice.) The installer won’t phone home to verify whether you are allowed to install.
The license allows you to install Lion on any computer you own or control for personal use. I assume that includes your wife’s machine.
For business use you can install Lion on all machines a single person uses or on one machine several people use.