I haven’t spent a lot of time delving into using Springpad for GTD, but I do know that there used to be an Springpad app that was specifically for that. It still exists, I believe, but has been removed from the main application for the moment. I mentioned this in one of the more recent Springpad posts (http://www.40tech.com/2010/12/08/springpad-easier-than-ever-to-save-and-organize-everything/), but didn’t actually give the link as it the old apps section may end up getting completely phased out. If you want to take a look, check out http://apps.springpadit.com/.
Also, you can send information directly to Springpad via email. Using the email address you signed up with (which can be changed in the Springpad backend), just send an email to the Springpad email that is associated with your account. You can find it in the setting of the web app.