Personally I think the wire mess on the PC is exaggerated. Here's what it looks like under my desk:
From left-to-right, those are VGA (monitor), DB-9 (mouse), Ethernet (LAN), DB-25 (printer), PS/2 (keyboard), RCA stereo (audio), and power. In particular, notice that they are not a tangled mess.
There are a couple of things I'd like to point out about this picture.
First off, it's a Dell. Dell provides monitors with built-in cameras, and nearly all Dell LCDs I've seen have USB ports built-in. Therefore it would only be necessary to run one USB cable rather than three from the tower to the monitor in order to hook up the webcam, mouse, and keyboard with less mess.
Second, they've apparently got a USB dongle for Bluetooth and/or Wi-Fi. To put the PC on a more level playing field with the Mac, it's worth noting that an equivalent internal card can be bought for about the same price as an external one. And as with the Mac, one could also hook up their keyboard and mouse through the wireless method as well, removing two more wires.
Third... Disconnected stereo cable. If there's one, there may be more, exaggerating the wire mess.
And to state the obvious... A more level comparison would've been the cables on a Mac Pro to the cables on a PC tower, since the iMac and tower are different form factors altogether.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:188.8.131.52) Gecko/20070726 BonEcho/184.108.40.206 (ayakawa SSE2-PGU)
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