Blink Geo asked the question after a post by High Earth Orbit (via Digital Urban who adds his own 2p worth).
IMHO it's necessarily a fuzzy line but overall, no.
Although you can turn layers on and off, pan and use a variety of tools, GMaps and GEarth are not really GIS in the old sense of the phrase as they are not fundamentally about data processing. A better way to think about it is by involving the idea of paper, in old pre-web days a GIS expert would analyse a problem, produce a set of map outputs and print them to paper. Now it may be better to 'print' the output to KML so it can be loaded to GMaps and GEarth*. The users are just looking at it, drawing conclusions maybe, but they aren't processing the data.
Fuzziness in the idea: Tools such as my thematic mapper and Map Tube are arguably allowing data processing within GMaps/Earth but certainly for the moment, you can't really call a collection of add on tools a GIS.
*This is not an original thought but I can't find the blog post where it was suggested.