Desktop focused products have been making slow and really clumsy forays into mobile but I've yet to see a mobile product translate poorly onto desktop...
It's much more about getting your design requirements listed in order of importance - if you spend the time thinking about questions like "if I have to fit this product/site/app onto a 4inch diagonal screen, what are the absolutely most important bits to show first?", you'll have a much easier time designing upwards to the tablet/netbook/laptop/desktop sized versions. Shrinking down to a 4" screen from the 1600pixel wide monstrosity that every single stakeholder has insisted has _their_ "critical" piece included is _way_ more work.
I've often used the "mobile first" approach to gently encourage clients to prioritize goals properly "I know we're not considering a mobile version of this site at this stage, but there's a _lot_ of conflicting requirements for your homepage here. If we were in the future to develop a mobile version, which of these things would we cut and which would we keep if we only had a phone screen to display everything on?" is an approach thats worked well for me…