Formula copy problem: only text value paste allowed

After years of trouble free usage with earlier excel versions, Excel 2010 is giving me the following copy/paste problem with formulas (Windows 7 OS).

I am trying to copy a cell containing the the following formula
=OFFSET(F200,1,0)-D200+E200
with the resolved value of
123.45
into a neighboring cell on the same sheet.

When I go to paste the selected function in the neighboring cell, the paste function only allows me to paste the resolved value of 123.45 as text, not the formula that I want.

Symptom: When I select the source cell, the copy selection I make has a solid background and not the expected animated dotted "crawling ants" boundary

Another symptom is that if I select one cell to copy and then a range to paste into, the paste only fills the 1st cell of the range (i get a "Data on the Clipboard is not the same size and shape as the selected area" popup)

If I try Paste Special, it offers two options "Unicode Text" or "Text"

Other info:
The text values that the formula is operating on were originally copied in from a CSV file.
But this is a excel file with a filename of test.xlsx and no other instances of Excel are open
System was rebooted and still exhibits this behavior.

Any ideas?