Yesterday I was re-creating my farm and ran into an interesting issue with BCS client functionality. I was deploying Microsoft Office and used Professional version of the build. I have an Excel Add-In project which was working fine before I redeployed Office. The VS solution complained about missing BCS client OM dll’s. I went to my C:\Program Files\Microsoft Office\Office14 folder and found the dll’s missing. I thought may be team has changed location of these dll’s in newer builds. Later I tried checking out the Professional Plus SKU to see if it’s a Office version issue. After deploying Plus, everything started working fine.
This basically means that BCS client features are only available in Microsoft Office 2010 Professional Plus SKU and you should be looking for version of your SKU while doing development.
This is my personal finding but may change by RTM so you can plan to wait for RTM and associated feature matrix to get released.