Rebecca,
You can ask your dev team to create a responsive landing page and it will work when served from Marketo as long as you don't hand-place any marketo widget (like a form, text, social widgets etc). The main reasons for this is that our designer relies on absolute positioning of the widget and this will break responsiveness of the template. Put differently, the template should contain all the HTML that you would want in the final Landing page. I know this will be disappointing, so I'll add that we are working on supporting responsive landing pages natively.
In the meantime, you could use Edward Unthank's Responsive Landing Page hack (
http://edwardunthank.com/blog/how-we-made-a-responsive-marketo-landing-page/), which templatizes using tokens
Raj