The hardware and system requirements of the update are detailed in iLife '11: System requirements support note. The first item in the list is a Mac computer with an Intel processor.
As far as I am aware, the last PowerPC Mac shipped in the summer of 2006. That's ancient history in computer generations, although many of these machines are still working away.
There are some other caveats in the iLife '11 hardware department. The software requires 1GB of RAM installed, which shouldn't be a problem even though many early Intel machines shipped with only 512M RAM. If they are still running (and most are), most must have been upgraded.
Other items in the hardware list are more puzzling.
For example, there's an item that says that the Learn to Play feature in GarageBand requires an "Intel Core Duo processor or better." As far as I am aware, there are no Macs with "less than" a Intel Core Duo, which was used in the first generation of MacBooks, MacBook Pros, Mac mini and iMacs that was Intel-ized. Perhaps they mean Intel Core 2 Duo?