No you do not need to sign up for a MobileMe trial to get onto iChat, nor do you need a credit card. The trick is to sign up for a .mac account and NOT a MobileMe account. The .Mac account is good forever for iChat (and iChat only).
Yup. The "additional information" on your referenced link is the way to go. I sometimes have troubles logging a newly created account in, however. Can;t recall what works when getting an "authentication failed" failed when logging in but try different networks from the pop-up: .Mac, AIM, or MobileMe with and without the domain extender for your new account.
32GB iPad 1 WiFi. 2011 Mac Mini Server (used as a workstation) 2GHz quad-core i7/8GB/1TB, 24" BenQ LCD, 17" NEC LCD, Magic Trackpad. MacBook 2.4GHz Core2 Duo/2GB/200GB/DL-DVDRW. Apple TV 2, 32" flat panel TV, Logitech DiNovo Edge BT keyboard & trackpad. >5TB of FW drives, 16GB iPhone 4S. In memoriam: my Sawtooth "Frankenmac" with upgraded dual 1.3GHz G4/2GB/360GB striped RAID/DVDRW/ATI Radeon 9000 Pro