While such software might be helpful for web designers, I would find it easier to test for Safari by using a bootable version of Linux with KDE and Konqueror on it. They are not the same, but Safari and AppleWebKit are based heavily on KDE's KHTML engine. You may not get the full picture, but you'll get a pretty good idea of what you're dealing with.
The only problem you might run into is configuring a network card -- Linux doesn't get along with some hardware as easily as Mac or Windows does. I've found that on four of the PCs I've tested a bootable Linux distribution on, you need to configure the network card manually. And, what if your distribution doesn't include the drivers you need? Well, you're just out of luck.
If you are using only client-side code (HTML, CSS, JavaScript, etc...), then you can always just load the files into Konqueror off your hard drive, which is usually mounted in /mnt/windows or /mnt/win_c. It will not work with PHP, ASP, or server-side languages of that nature, unless you can find a bootable distribution with Apache (which I would find highly unlikely).
MSIE for Mac is a different story, since its rendering engine only exists on Mac and no other operating system. Mozilla is completely cross-platform, so it will look exactly the same under every operating system (with the exception of installed fonts and video card color settings).
UserAgent: Mozilla/5.0 (compatible; Konqueror/3.1; Linux 2.4.22-10mdk; X11; i686; en_US, en)
Laptop: HP Compaq nx6325 - Turion 64 X2 @ 2GHz, 2GB DDR2, 100GB HD, ATI Radeon X300, 15" LCD, Seven Pro
Handheld: Palm Treo 650 - Intel PXA270 @ 312MHz, 10MB RAM, 32MB flash, 2.7" LCD, Palm OS 5.4
