OK, this is a pretty specific topic, but I may not be the only one who's wanted to figure this out.
(I wish Google would make their cool logo for Google Docs available, as it would have spiced up this post. Oh well. See it here: http://googledocs.blogspot.com/.)
Here's the scenario: I'm helping Vicki Davis and Julie Lindsay set up a workshop this summer to teach their "Flat Classrooms" program to other educators. I'd set up the website as a Wikispaces wiki, with a custom domain. I wanted to use Google Spreadsheets new form capability (very sweet) for registrations--that way Julie and Vicki and I could all see the registrations as they came in. The trouble was, even though you can easily find good information on how to set up a form and collect information, and then even how to embed a form using the HTML "inline" function into another webpage--it wasn't working in Wikispaces. Using the aforementioned methods to add code in the "Other HTML" option from the "Embed Widget" button, I was getting a "404 Page not found" error.
So I read around and something clued me into the fact that when you set up a form in Google Spreadsheets, you can have the form emailed to you. In Gmail I went to the "show original" option while in the actual email that I received, and I could see the web-page code that had been generated to show the form in the email--which was regular code for an HTML submit form, and not an inline link to the form page. Well, that may be Greek to you or not, but all you need to know is that if you go down to the part of the original email that starts with the left triangle bracket before "form action="http://spreadsheets.google.com/formResponse?key=..." and you highlight through "/form" finishing with the right triangle bracket (which is almost at the end). You can now paste that code into "Other HTML" from the "Embed Widget" function, and your form will appear (and work!) on Wikispaces.
What I actually did after I discovered this would work is that I pasted the code into a text document, and then added additional "breaks" to make it look a little nicer before pasting into Wikispaces. But that's for nit-pickers... :)