I just finished building a 7 testing platform, so have this a whirl. I assume you mean SSAS when you say OLAP (vs Essbase?)
I created a basic report off of the Sample AdventureWorks cube, and had no problems "Downloading all rows as text file" as you describe. However, I created the report in Desktop 7 vs. 6.X so we're not exactly dealing with apples and apples.
If you download one of these 6.X workbooks currently in your 7 Server, save / upgrade it to 7 using Desktop 7, and then re-publish to Server 7, do you see any change in behavior?
Thanks for your efforts Russell. Glad you were able to get it to work. I did open a case with Support.
Some other information that may be relevant...
* all of our dashboards on our server were converted to 7.0 in desktop then republished to server.
* Our server box is Windows 2008 Server, SP2
* Users that we know of are using Windows 7 on their PCs.
* An error we are also seeing sporadically is "Additional Considerations* Tableau Server is no longer supported on Windows XP and Windows Vista operating systems.". I personally only saw this error in Chrome when I used tools "Edit" and "Paste" after clicking the "download as a text file". Others users with IE are getting the error pop up in a separate tab.
Oh, one other item....the download to text works when clicked from the Summary tab. It does not work when clicked from the 'Underlying Data' tab.