clearly the plugins wont work, its a completely different product "under the hood"... the upgrade will first uninstall avast4 then install avast5, therefor losing all plugins..
the architecture for Win7 should be simular to Win2k8 (r2) but thats without plugins....
there for i assume that the business products that will be released later with plugins will be incompatible, besides the engine/virusdefinitions, with the home version and therefor the home versions would only provice basic protection on a Win2k8 server
edit:
instead of speculating i took actions.....
i downloaded the installer for the most advanced avast package and setup a win2k8 server. I started the installer and it direct came with the following text:
Your operating system does not meet the minimum
requirements. You can install avast! only on Windows
2000 or higher. It cannot be installed on Windows
Server
this should make an end to your question

and according to the logfile the OS is not even recognised correctly as i suspected (see the bold text, i am running Win2K8 R2):
26.01.2010 21:03:50 general: Started: 26.01.2010, 21:03:50
26.01.2010 21:03:50 general: Running setup_ais-179 (377)
26.01.2010 21:03:50 system: Operating system: Windows 7 ver 6.1, build 7600, sp 0.0 SERVER x6426.01.2010 21:03:50 system: Memory: 86% load. Phys:102992/785912K free, Page:210628/1834488K free, Virt:2035180/2097024K free
26.01.2010 21:03:50 system: Computer WinName: BOZW-EXCH-01
26.01.2010 21:03:51 system: Windows Net User: BOZW\Administrator
26.01.2010 21:03:51 general: Cmdline: /sfx /sfxstorage "C:\Users\ADMINI~1.BOZ\AppData\Local\Temp\_av_sfx.tm~a03824" /srcpath "C:\Users\ADMINI~1.BOZ\Desktop" /sfxname "setup_ais_eng"
26.01.2010 21:03:51 general: DldSrc set to sfx
26.01.2010 21:03:51 general: Old version: ffffffff (-1)
26.01.2010 21:03:51 general: Install check: SetupVersion does NOT exist
26.01.2010 21:03:51 general: SGW32AIS::CheckIfInstalled set m_bAlreadyInstalled to 0
26.01.2010 21:03:51 registry: Get registry: Software\Microsoft\Internet Explorer\Version=8.0.7600.16385
26.01.2010 21:03:51 general: Err:Your operating system does not meet the minimum requirements. You can install avast! only on Windows 2000 or higher. It cannot be installed on Windows Server.