No. You can download the developer preview, make yourself a recovery USB stick by "restoring" InstallESD.dmg to an 8 gig or larger USB stick formatted HFS+/GUID partition scheme, then boot to that stick by holding ctrl to install ML. I upgraded SL to ML using this method on my test machine and it worked just fine.
BTW, if you redeem a code for ML and the download fails b/c you aren't on a supported machine, don't panic. Simply log in to the app store on a supported Mac and download ML from "purchased items."