An interesting comparison from this post in the Arstechnica.com forum:

Year: 2000
Model: iMac
Operating System - Mac OS 9.0.4
Processor - 500 MHz PowerPC G3 CPU, 128MB Memory
Graphics - ATI Rage 128 Pro, 8MB of memory (8 million triangles)
Screen - 786K pixels
Data Transfer Speeds - 1.3-12.5 MB/s (DVD-ROM-1/100 Ethernet)
Storage - 30GB Hard Drive
Dimensions - 15.0 x 15.0 x 17.1 inches
Weight - 34.7 pounds

Year: 2010
Model: iPhone 4
Operating System - iOS 4.0
Processor - 1 Ghz ARM A4 CPU, 512MB Memory
Graphics - PowerVR SGX 535, uses system memory (28 million triangles)
Screen - 614K pixels
Data Transfer Speeds - .04-20MB/s (3G-WiFi)
Storage - 32GB Flash Drive
Dimensions - 4.5 x 2.31 x .31 inches
Weight - 4.8 ounces

Numbers/stats are neat, but one crazy thing to think about is that the iPhone 4 can record and edit HD (720p) video - which you would never be able to do on that desktop iMac.