Agreed. I think Lion is just the first step in the integration between OS X and iOS. I don't think the two will ever be combined into one product but I do see them becoming extremely similar with iOS just being the lightweight version of the two.
... or Mac OS being the lightweight version of iOS, no?
No.
OS X and it's future will still be needed to do all the heavy lifting of computing, i.e. video/audio/photo editing, CAD, 3D graphics, database entry, complex spreadsheet work etc... iOS isn't built or intended for this line of work. It can do some, but only as a "light" version.