The short answer is to use the "mobile" google URL's (the ones with the "m") meant for phones. The regular google pages won't work for google spreadsheets.

https://bugs.maemo.org/show_bug.cgi?id=2337

Reading between the lines of the bug, I think that google spreadsheet doesn't present the spreadsheet cells as input fields, so there is no way to know to pop up the virtual keyboard.