It all depends on the EULA. Just because a piece of software doesn't require a serial number or explicity check for other copies running on other machines doesn't mean it's legal to install it on more than one machine.
I don't have iLife '05 yet, but Apple's EULA for iLife '04 explicitly states that the software can only be installed and used on one computer at a time:
This License allows you to install and use one copy of the Apple Software on a single computer at a time. This License does not allow the Apple Software to exist on more than one computer at a time, and you may not make the Apple Software available over a network where it could be used by multiple computers at the same time.
From:
http://www.apple.com/legal/sla/ilife04.html
In my interpretation, that pretty much says that you can install the software on only
one machine. Even if you only used one copy at a time, it would still be illegal to have the software installed on more than one machine -- so you home/office users that have a desktop and a laptop will need to purchase the family pack, or multiple single-license copies of the software in order to use it on both machines.
I'm pretty sure the EULA hasn't changed much from iLife '04 to iLife '05 -- can someone with iLife '05 take a look at the EULA and see?
Adobe's EULA does permit home users to install the software (PhotoShop CS) on up to
two computers, as long as the software is not used simultaneously.
http://www.adobe.com/products/eulas/main.html
Macromedia's EULA is similar to Adobe's: permitting home users to install on a maximum of two machines, so long as the installs are not used simultaneously,
and only if the license holder is the one using the copies (so if you install on two computers at home, only one license holder is allowed to use the software -- so if you install on your computer and Dad's computer, Dad can't use the software
at all. Only you can.)
http://www.macromedia.com/software/eula/tools/
Microsoft's is a little more convoluted. They have different licensing restrictions depending on the use: for example, someone who uses Microsoft Office at work can obtain a license to use the software at home as well, but only under certain conditions. It's not a surprise to me that Microsoft has a very strict and complicated EULA covering all types of uses of their software...
At any rate, it seems that Apple is more strict with their licensing, permitting only one copy to be installed and one copy to be used; whereas other companies are a little more lenient, allowing the software to be installed on two computers in certain situations.