Hey Jenn,
You can trigger Marketo campaigns via SFDC form fill outs. One down side to this is that any action desired will be in tune with the sync between SFDC and Mkto. For my company, we slowed the sync to every 30-35 minutes, meaning if there is a form fillout 4 minutes after the last sync, then any qualifying campaigns will not be triggered for at least 25 mins. May not be that big a deal, but just something to consider.
Depending on the fields you have in your form, you build a trigger campaign in SFDC using the "Lead is Created" trigger. Then, in the contstraints, you can determine which Lead creating activity qualifies for the campaign. You can use other filters to further restrict campaign qualification. You can also trigger campaigns if the Lead existed already. Be careful with this because, if my memory serves me correctly, SFDC forms do not auto-populate so you can be creating duplicate Leads for the same person.