This appears to be a well known issue, but I just ran into it with a customer of mine. It appears that when you insert a custom list form in SharePoint Designer, it doesn't have the correct objects for attachments. Check out the following article for the fix.

http://support.microsoft.com/Default.aspx?id=953271

You will need to install a hot fix, make sure you installed SharePoint SP1 first, and then update the code on the pages.

A few of my own notes
- Pay attention when you're pasting the code samples. I think SharePoint Designer converted the quotes in this article to the HTML version: from " to " so you'll have to go through and change them back.
- To close the webpart in WSS since Edit Page is not available as it is in MOSS, change the value <IsIncluded>true</IsIncluded> to false. You will still see it in SPD, but it'll be grayed out.

HTH