Do I understand you correctly, that you are looking for a portal solution, integrating the apps ? Or do you look for an "application supplier app" or do you want to reimplement the RIAs with a common framework ?
Without much hassle (especially with your skills) you can
* use Alfresco to develope (or rather adopt) frontend pages integrated into your Alfresco GUI
* use Flex, ExtJS, PHP clients, which access alfresco storage, security model, search framework, library services
* even use Alfresco Plattform to reimplement your applications (especially with documentcentric apps)
* use Alfresco Share to set up social collaboration sites a la MOSS sites
But neither Alfresco nor MOSS are a portal system. And yes, you can do the above with MOSS as well, but I think, not wich ExtJS & PHP skills, so not only from softwarestack, but also from skill stack, Alfresco seems more fitting.
Norgan