So in testing IE8 at my company, I and several others encountered numerous problems trying to access the Altiris NS 6.0 console with it. In fact there was a recent forum thread with a similar issue. When accessing the console, we would receive an error similar to the following:
From what I've seen, this usually occurs because the "User Agent" string for your browser is too long. Apparently the code that does the browser check can't handle it and errors out (maybe it is exceeding some buffer). In our case, we had remnants of our configuration which was managed via the IEAK from IE6 which had added some "custom branding" text to the User Agent, along with several different versions of .NET Framework which add to it, ALONG with some additional headers added by MS Office InfoPath 2007. Combined all together the User Agent was well over 200 characters long. You can check and analyze your own User Agent string here.
Check the registry at the following locations: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform
See the following MS KB article also which has some additional information about how the User Agent string is created: MSDN article.
Despite information to the contrary (AKB 46279), IE8 seems to work fine with NS6 console as long as you enable "Compatibility Mode". You can enable this using the "broken page" icon next to the Address bar. There are some small incompatibilities (I've noticed some scrolling issues when selecting a package in Software Delivery) but overall it seems to be OK, but of course YMMV.