Once you've installed Office Web App on your Sharepoint 2010 Beta 2 (on a single server farm in my case), you may encounter the message "an error has occurred" when you try to open an office document in Office Web App.

Don't forget to activate the Office Web App feature on your site collections if you want to avoid that :

• Go into the "Site Actions" of your site collection

• In the "Site Collection Administration", select the "Site collection features"

• Activate the "Office Web Apps" Feature :

And have fun with this very nice feature!