You can connect an iPod to ANY receiver. You can use any analog input on the receiver - CD, Aux, Tape, the audio portion of Video 1, 2, 3 etc.
Receivers that advertise 'iPod compatibility' simply offer additional features such as the ability to scroll the name of the song on the display and the ability to control the iPod functions with the remote that comes with the receiver. Most music/HT enthusiasts will have a universal remote anyway so I feel that is no big 'feature' at all.
I am very familiar with Onkyo products as that is what I have always owned. Onkyo's dock provides some addtional features that you may or may not find useful and I'm sure some of the brands offer something similar. For example, the Onkyo doc utilizes their RI (Remote Interactive) feature. If you turn on the iPod, the receiver will automatically turn on and change the input to the one where the iPod is connected. If you then turn off the iPod, the receiver will automatically go into standby mode. RI provides those features and others for other components too. So in some sense it is a nice convenience feature but one you can easily do without.
iPod compatibility may be a nice feature to have but is certainly not a necessity. I would pick a receiver based on other factors such as power, ease of use, and price point because you can connect an iPod to any of them - you would just be lacking the so-called convenience features if it is not 'iPod compatible'.