Or, perhaps more importantly, their $200,000 signing bonuses? That’s the question Dahlia Lithwick takes on in her recent Jurisprudence column for Slate.
The sums in question are even larger than Lithwick notes. She writes:

That will be [a] $200,000 [bonus] on top of a starting salary of $145,000 to $160,000. Which adds up to an awful lot of Pottery Barn sectional furniture for someone who is, on average, 26 years old and just two years out of school. As Chief Justice John Roberts pointed out recently, that $360,000 beats the heck out of the $212,100 he’s taking home for, well, chief justice-ing the entire nation.

Actually, the starting salaries are even higher, since pretty much all firms give Supreme Court clerks seniority credit for their two years of clerking. So a clerk who went straight through to a feeder judge, the SCOTUS, and a private law firm would be paid like a third-year associate: $170,000 in Washington, or $185,000 in New York (or in the D.C. office of a New York firm).
Lithwick interviews Walter Dellinger and Carter Phillips, who offer various justifications for the outsized bonuses as an economic matter. We have our doubts — and are quoted as a dissenting opinion:

On his legal gossip blog, Abovethelaw.com, David Lat tracks lawyer salaries with the glee most of us reserve for American Idol. And according to him, the hefty law clerk bonus stopped making any real economic sense several decimal points ago. Lat notes that these new associates just don’t bill extraordinary hours; that boutique appellate practice isn’t that lucrative; and a good many former clerks have academic aspirations. “They’re billing 1,800 hours, not 2,500, and a lot of them are probably already working on their job talks,” he says, referring to their sales pitches for the academic market.

The real allure of the Supreme Court clerk, says Lat, is that they are trophy purchases, “something for a firm to crow about in their recruiting materials.” Ouch. If Lat is correct about this, the boutique firms are buying former Supreme Court clerks when they might be better off investing in something more enduring, like new leather sofas for their lobbies.

We stand by these remarks, but maybe we’d remove the “Ouch.” These bonuses don’t make pure economic sense (in our opinion); but neither do many other things that law firms spend gobs of money on. If a firm wants to drop $200,000 on a SCOTUS clerk, or on an Alexander Calder for the lobby, that’s their prerogative.
We’re quoted later in Lithwick’s piece:

[S]ome firms, notes Lat, have decided to stop pursing the Supreme Court clerks and spend their recruiting dollars on what he characterizes as the near misses. “For every one of the 36 smartest law kids,” he says, “there is another equally smart law kid who just had a bad interview [for a Court clerkship].” And if law firms make the economic decision to give bonuses to them, “they get all the benefits of a knock-off Prada purse: They perform the same function, they look great, and you know they’ll do a great job.”

We’d single out Kellogg Huber of D.C. as one such firm. Some of you have expressed curiosity about who pays the biggest clerkship bonuses. We believe it’s Kellogg Huber. This tiny, super-elite Washington litigation boutique is rumored to pay clerkship bonuses of $100,000 to federal appeals court clerks — and for that kind of money, combined with the firm’s small size, it can afford to be picky. The non-SCOTUS clerks at the firm tend to be those who came thisclose to landing a job at One First Street (e.g., feeder-judge clerks who interviewed unsuccessfully for Supreme Court gigs).
Update: Do you have an opinion on whether Supreme Court clerkship bonuses are too high, too low, or just right? You can express it by voting in our poll. To vote, click here.
What to make of those astronomical Supreme Court signing bonuses? [Slate]