The fascinating thing about this question is how much of a moving target it really is. Even just a few months ago, for example, we might have included service offerings like solar financing plays as something VCs likely wouldn't invest in, because of the lack of defensible intellectual property and low barriers to entry (very generally speaking, of course). Now it's one of the hottest investment areas, as the rapidly-growing solar market has led to expectations that the market can support multiple "winners". Low-tech plays in biofuels and other hot sectors have been getting plenty of capital, too, even if not always from VCs (although, sometimes...).
So what's out of bounds? The very fact that this question is being seriously asked reflects a rapid expansion of investor appetite that is at the heart of the recent concerns about a cleantech venture capital "bubble", as we've discussed before. And, as we've described on this question, while there doesn't appear to be any "bubble" across the entire sector, certain subsectors and market niches can certainly feel frothy. Among other factors is the simple truth that not all "hot" investment areas will end up providing the returns some investors are currently expecting, so philanthropists can't consider a societal problem solved just because VCs are putting early money into potential solutions...
So Carla, my two cents' worth as of this moment in time would be: Put your money into the demand side of the market.
In other words, VCs and other institutional investors (project finance, public markets, etc.), not to mention government and academic research efforts, seem to be very happy to put money into the supply side, funding a rapidly growing list of cleantech technologies and services. I hesitate to nominate any particular technology or service sectors as being out of bounds for VCs, because that answer will continue to change as market conditions (and investor optimism) change. But without the anticipated growth in demand for these products and services, a lot of these investments will inevitably fall flat.
Water tech is a great example: There are some great technology development efforts underway that would clean up the water we drink and the wastewater we create, but the most likely buyers aren't buying. And there are a lot of energy efficiency options out there for homeowners and commercial/industrial building owners that make stunning financial sense, and yet still haven't been widely adopted like you'd expect. There are major segments of most clean/green technology markets that VCs and others still aren't putting their money into, but most of those un-addressed value chain segments tend to be downstream.
The most important thing philanthropists could do, in other words, would be to prime the engine of clean technology market adoption. Instead of catalyzing technology development, catalyze market development. Fund adoption of clean drinking water and solar and other distributed energy techs in emerging economies. Educate U.S. water and electric utilities (and their regulators) about the various new choices available, and how they can provide compelling economics and better quality at the same time. Be an angel investor for a local energy efficiency consultancy or green home product distributor. And help consumers understand their green and clean choices, empowering them to actually adopt the solutions that are in their best interest, but that they either don't know about or don't realize the economics around.
And thanks to Carla for asking a great question -- there are probably a lot of great ways philanthropists and investors can work together "profitably", by both sides' definition of the word...
Readers are encouraged to add their own comments and suggestions for Carla as well.