From the previous post about OpenSSH banner on Ubuntu, I just found the easy method to hide the distribution version in Ubuntu 10.04 by chance.
The Ubuntu 10.04 has the new option for sshd_config. That is "DebianBanner". You just have to add "DebianBanner no" line (without quotes) into sshd_config, then restart it. Your OpenSSH banner will change from "SSH-2.0-OpenSSH_5.3p1 Debian-3ubuntu3"
Normally, Ubuntu ports code from Debian. So I checked the option in Debian as well. I found the option in Debian 6 as well.
PS: The option has been added since Ubuntu 10.04.