So I followed Rob’s advice and investigated carbon offset providers and their projects.  My research wasn't exhaustive - I started with a Google search and then made a table of the companies that caught my eye.  All of the below options are verified by a variety of outside bodies, such as the Environmental Resources Trust.  (From what I understand, in this new industry, verification isn’t required nor standardized, so this alone isn’t an adequate measure of the quality of offsets purchased.)

TerraPass - calculated 41,423 miles flown, 16.5 tons of carbon emitted, $74.25 to offset
TerraPass was the one company I had heard of before, so that’s where I started.  I felt I learned the most from its job listings corner; while I like their enthusiasm for the for-profit model - I've worked in nonprofits, I know how little they can accomplish (though not all of them) - I don't like this little quote about products:

You are turned on by the idea of working with consumers. We sell stuff. Do you like to talk to customers? Do you have a good head for creating products that people will love?

They have a whole shopping center on their site, with icky throw-away products!  And they give you crap like stickers and bag tags if you buy their offsets! (You can op-out, though.)

In more substantive matters, and bad press aside, I felt like their projects were a little high on the technology side and a little low on the renewable side (perhaps this is a misperception?)  Their project portfolio also hasn't been updated in over a year.

Carbonfund.org - calculated 40,000 miles flown, 16.7 tons of carbon emitted, $41.80 to offset
This one seems a little nonprofitty in that their website isn’t as slick and organized as TerraPass.  I like the local focus of their projects and the wider body of verification organizations looking after their work. They support reforestation projects, which I understand aren’t that well-respected in terms of carbon sequestering or local sustainability.

Sustainable Travel International - calculated 41,000 miles flown, 18 tons of carbon emitted, $286.69 to offset
STI seems like it falls somewhere between Carbonfund and TerraPass in terms of organizational infrastructure: their website is a little nicer to look at though a little confusing.  I like that they have a spreadsheet comparing many of the products out there; TerraPass and Carbonfund aren't on the spreadsheet - which were the first two companies that came up when I googled “carbon offsets.”  The spreadsheet also contains the three projects it primarily invests in...a little sketchy.   [Update: I can’t find the spreadsheet anymore on the STI site – if you’d like to see it, email me and I will send the one I downloaded a few weeks ago.] One of STI’s three projects is very concerned with local jobs and local impact on the environment, which I like; another is Conservation International – and biodiversity is something I feel strongly about.  Plus two of their three projects are the American branches of European models and those Euros seem to be much better at not taking up so much space and resources!

Native Energy - calculated 41,667 miles flown, 16.6 tons of carbon emitted, $204.00 to offset
I like the projects Native Energy supports, mostly wind farms and methane-capture projects on family-owned dairy farms.  The for-profit company is partly owned by an organization of Great Plains Tribes, who have tremendous wind resources – it seems like an excellent money-making scheme to me.  Their website makes me think they have a very hands-on approach, which I like; I also appreciate their commitment to communities in need, like small farmers and Native Americans.

FlyNeutral - calculated 41,438 miles flown, 18 tons of carbon emitted, $135.00 to offset
FlyNeutral is an interesting enterprise: it’s a project of the Presidio School of Management, a business school.  They work entirely with the Chicago Climate Exchange, using credits to fund large-scale sustainability projects.  I interpret their model as largely theoretical and economic, which, when compared with the financial models that make the rest of the world run, aren’t so abstract after all.

So...what do you think?  Obviously there are a lot of choices...I have a feeling, though, that regardless of which offsets I buy, it is largely a symbolic gesture of responsibility on my part.  It seems that this is a little tit-for-tat when what really needs to change is policy and commerce, and in a big way.  

The way I see it, the offsets model leads to cheaper alternatives to dirty energy, which will make the cleaner alternatives grow and get incorporated into the existing infrastructure.  Which is good.  The endpoint, though, doesn't make sense: at a certain point there will be a stalemate between the very dirty energy like private jets (and even air travel in general) and the cleaner energy, which isn't perfect, of course.  There will be no more energy to offset, unless we encourage people to use more energy. Clearly this is far off, and maybe impossible, given the growth of the population and the explosion of developing nations; however, it seems that offsets are a more likely a stopgap solution for this trendy green moment, making people feel a little more "responsible" without having to examine their desires or needs.