New Google Docs and Opera
Update: This workaround is no longer needed with Opera 9.50 which seems to solve this problem. Check your version and update if needed.
Google has recently developed a new Google Docs version and if you thought they took the chance to improve (at least a little) the compatibility with other browsers, you couldn't be more mistaken. Actually, it went worse with Opera.
If previously it was enough by adding ?browserok=true in order to enter Google Docs with Opera and mask it as Internet Explorer to use Spreadsheets, now it gets constantly reloading with Google Docs and Spreadsheets is only usable with a poor read-only interface (masking Opera as IE throws errors).