John Gruber of Daring Fireball, a major Apple/fan/critical site, reports:

[...] after accumulating the 5,000th response, the survey I set up yesterday with Google's new spreadsheet forms feature [ http://daringfireball.net/linked/2008/february#wed-06-survey] crapped out and began generating errors when you attempt to submit the form. (Looks like 5,000 rows is the limit?) [...]

http://daringfireball.net/linked/2008/february#thu-07-survey_update

Each spreadsheet can be up to 10,000 rows, or up to 256 columns, or up to 100,000 cells, or up to 40 sheets – whichever limit is reached first.

http://documents.google.com/support/spreadsheets/bin/answer.py?answer=37603