Ok, just for grins, I went to dell and apple, and configured similar systems. I upgraded the mini to 160gb ( the largest drive they offered, still 90gb short of the 250 on the dell, but a pretty good size, but I agree it's smaller.) I added the usb mighty mouse and keyboard. It was $797.
Then I went to dell. Started with the cheapest base inspiron ($349). Added wireless, Windows Vista Ultimate, Roxio DVD burning software, upgraded to the core 2 duo 1.83ghz chip that's in the cheapest mini (yes, that's right, the stock dell is slower than the stock mini), and 30 days of phone in technical support (you get 90 stock with the mac). The total was $747.
The differences between the systems:
The mini only has a 160gb hard drive, doesn't burn dvds in the lowest end configuration, but does include ilife (garage band, imovie, idvd, iphoto, etc), a firewire port (I could have added one to the dell for $30, but figured most people wouldn't), bit perfect digital out (A $30 card for the dell you'd have to buy elsewhere and add to the dell).
The dell had to be upgraded to vista ultimate, because the rest of the versions of vista are seriously crippled, does have 250gb and a dvd burner.
They're very similarly useful systems. You pay slightly more for slightly less with the mac, but get a more elegant package, a better keyboard and mouse, and OS X instead of vista. I didn't include the cost of a virus checker, but the average home user probably would buy one from dell.
If you go to the all in one configurations, the imac and the dell xps are almost exactly the same, for the same price, $1299. Only with the dell, you don't get an uncrippled operating system.