After scoping out the wonderful advice here and on other forums, here is what worked for me.
OS: Windows 7 Ultimate 64
Office 2010 Pro Plus x86
DM: 5.01.37
This was a clean install of Win 7 & Office on a new partition (dual boot to XP/Office 2003) and I used BackRex Outlook Backup to transfer my account info and settings to the new install.
Initially, of course, I was unable to sync my 8900, receiving the same "Message Store not found" message when trying to configure the sync settings.
I decided to try the (presumably) easiest solution first - installing OL 2007 without uninstalling OL 2010.
I was able to install OL 2007 and configure sync, but when I tried to sync, got "Function OpenMsgStore" error. Googling this pointed me to the following solution:
Cause
Either the Messaging Application Programming Interface (MAPI) profile or the Intellisync configuration is invalid.
Resolution
Reset the Intellisync configuration by completing the following steps:
1. On the computer, open BlackBerry Desktop Manager.
2. Double-click the Synchronize icon.
3. Select the Configuration tab.
4. In the Advanced section, click Reset configuration.
Create a new MAPI profile through the Mail icon in the Windows Control Panel.
Basically, what I did was go through Control Panel>Mail, copy my profile to a new name, then delete and add back in the one MAPI email account I had.
After rebooting, I was able to run sync, with one error, which apparently self-corrected the second time I tried.
Now for the real test, I tried to reopen OL 2010, but it wouldn't start due to having the wrong version of olmapi32.dll (installed from OL 2007). I did a repair install of Office 2010 to overwrite that back and now BB syncs and OL 2010 works.
Haven't yet tried to reconfigure DM sync, but don't really need to at this point. Not sure if I can now uninstall OL2007 now or not?
Edit: It appears that one MUST uninstall OL2007 after setting up sync as otherwise Microsoft update will again overwrite the olmapi32.dll that OL2010 needs to work. After running MS Update and rebooting, my OL 2010 would no longer work, so I uninstalled OL2007, rebooted, did a repair install of OL2010, and sync still works.
Now I think I can put my hoops back in the closet for a little while.