Sounds really nice, right? But there are apparently some pitfalls with that...
Peter F. Hamilton.
(Disclaimer: i am in no way endorsing the research below, because i simply have not looked into it in any detail.)
The idea apparently has some merit, although it seems a bit far-fetched at the moment (see the new scientist story below). From this perspective energy sources that are driven by the sun (solar, wind and tidal power) is much more 'clean' than those generating heat through more unnatural processes (i.e. nuclear fission and fusion). If we ever get a cheap infinite supply of carbon free energy, then you would imagine that the energy consumption would explode. Probably enough to make the Earth warm considerably.
There is one very important difference between heat dissipation warming and greenhouse warming. Greenhouse gases will remain in the atmosphere for several centuries and continue to warm the earth even after we stop generating more. The warming from waste heat will quickly get lost to space if we suddenly reduce our consumption.
New Scientist: Heat we emit could warm the Earth.
Cowern & Ahn: Thermal emissions and climate change: a nuclear problem and a photovoltaic solution?