Yes, I can say that the rumours are true: the beta version of Windows Live Messenger 8.0 suffers of a terrible memory problem.
If you try to open your MSN 8.0 beta and you check your Windows Task Manager for the msnmsgr.exe process, you can see that the memory usage grows indefinitely with every window you open and when you're on a conversation.
I don't know what could be the causes, maybe the internal debugging code (that I'm sure is still present on the beta) or something wrong on the internal code, but the problem is true and present.
Microsoft, remember this and correct it for the next releases... I don't want to have a Gb of RAM only to talk with my friends! 
Update: Thanks to Romeo Pruno that has leaved me here a good feedback: check this folder for another terrible surprise:
C:\Documents and Settings\[profile name]\Local Settings\Application Data\Microsoft\Messenger\[Messenger signin name]\SharingMetadata\Working
The size of this file could be enormous. Another problem, maybe connected to MSN Sharing Folders.