Did a quick search here and it would seem Marketo does NOT offer responsive landing page templates. Please tell me I'm missing something ...
Here you go - some guided landing page templates: Guided Landing Page Templates - Marketo Docs - Product Docs
If you're talking about free form landing pages, see if the freeform editor can work it's magic and automatically make the page responsive.
coding pages is largely the buyer's responsibility. Marketo offers some free ones and there are free ones from Knak, Grazitti, and others. Usually ideal to get a coder to do this for you.
Does anyone have any examples of some great, responsive marketo landing pages that have been coded by the users? Or any sources as to where to find that kind of information? DJ Waldow?
Generally, the free-form landing page templates are not responsive "out-of-the-box".
You might have better luck with some of the Guided LP templates, but the library is no doubt limited. Bootstrap is a free framework you can use with the guided landing pages to jump-start your Responsive page-production (http://getbootstrap.com/ ).
@Stephaine - As Josh mentioned, there are vendors in the space working to bridge this gap, and it's a shameless plug, but I know of at least one more I just did a "Show and Tell" presentation on the framework we've developed for Guided Landing Pages and Email 2.0, you can check it out here: Digital Pi .