From Paterva Wiki
Maltego version 2 has been tested on Windows XP/Vista and Linux (various distributions). As Maltego is purely Java based it should work on almost any operating system. Because operating systems differ care has been taken to use an Install Shield (Izpack) that takes of differences between these systems. If you have successfully installed Maltego on operating system other than Linux/Windows, please take the time to let us know so that we can update this list. If you are unsure if Maltego will work on your operating system please Contact Us before you buy.
Bottom line: Windows XP or Vista and Linux works fine, others should work too, but you might need someone more technical to assist you. Mac users see 'Software requirements' below.
Maltego uses Java version 6 (1.6) which is available for most popular operating systems. Maltego v2 will not function correctly with version 5 (1.5). The Maltego installer will not install or upgrade your system to Java 1.6, but this should be a painless procedure. The latest release of Java can be downloaded for your operating system here. This page also includes instructions for installing the software. Note to Mac users - Java 6 is not officially available for Macs yet. However - a preview of Java 6 can be downloaded here. When version 6 is officially supported we will begin doing testing on it. If you have trouble installing Java 6 then you should ask someone technical to provide you with assistance.
Bottom line: You need Java 1.6 and you need to install it yourself...but - it's easy to install. Technical Mac users should give it a try (you can get it work) but it's not official supported.
Update - forum post here explain how to install Maltego on OSX 10.5.
Maltego loves memory and raw CPU power. Rendering views take a lot of computing power and the slower your computer, the longer it will take. If your computer is under-powered this can become frustrating, but things won't break. If you plan to work on large graphs you'll also need some memory. Maltego version 2 is configured to use 512MB of RAM, but if that is all you have your OS and other apps will have nothing left to work with. We thus recommend at least 1GB of RAM, but the more the merrier. You also need a link to the Internet if you want to use the Paterva TAS (and for registration). Almost all the data collection and processing happens on the server but the results still need to get to your computer. A fast Internet link makes Maltego work faster. Lastly, if you ever needed a reason to get a big screen you now have it. Maltego also loves big displays. Running it in 1024x768 just wouldnt feel right - but you can do it if you really have to. If you unsure if your computer will run Maltego please Contact Us to find out - or at least get an opinion.
Bottom line: Minimum (ouch): 1GB RAM, 1.8GHz, 64Kb Internet access, 1024x768 display. Recommended: 4GB RAM, 3.0GHz dual core, 1Mb+ Internet access, 1600x1200 display.