Max -- As the founder of a company that markets law firms on the Internet (as well as the author of the referenced article about LinkedIn), I can enthusiastically vouch for measuring ROI when it comes to law firm marketing. In fact, any firm that markets _without_ measuring ROI is, as a pilot would say, "flying without instruments."
Granted, off-line, non-Internet marketing efforts are more difficult to measure, but my firm only markets my clients online, where every click, pageview, filled-in form, and referring search query is tracked.
Many of my clients do just the type of ROI tracking of which you may be skeptical -- they review after a year or a quarter what business came from Internet sources versus the money they spent on bringing in that business. Most of my company's clients see four, five, and higher returns on each dollar invested this way.
LinkedIn, while a new tool in our toolbox, is merely another way of getting potential clients to your website or front door or telephone line.
And, of course, you should always, always query a new prospect on how he heard of you.
JPL

I believe LinkedIn memberships are just up across all professions. I've noticed an increase in LinkedIn requests the past couple of weeks--and none came from other lawyers.
I think another side-discussion is whether you would want to link up to clients. Not that I'm paranoid, but I just don't feel like disclosing my client roster to the general public. Unless, of course, they all recommend me on the site.

I'm always skeptical of ROI as applied to law firm marketing. Unlike, say, toothpaste, we can't point to a particular campaign and then follow the rise or fall of sales. The retention of clients may, at the end of the day, look like a bell curve, with more clients going to the firm's that plaster their names on the sides of transit buses, and fewer going to the solo in his office with no website, but the client retentions themselves follow Poisson distribution, i.e., with each event fairly unconnected to another.
LinkedIn is, to me, similar to a blog: it's another part of "presence," showing the random lawyer name to be an actual person with actual experience and connections. potential clients can get a sense of what a lawyer is like from reading their blog, and a sense of how the lawyer fits in with the business community by looking at their LinkedIn profile. I don't think you'll be able to quantify that well, except to note that most users don't know/care about it but, every now and then, it really mattered to someone, if they know it or not (keep in mind most customers vastly underestimate the the impact television advertising had on them).

Kevin- Thanks for your comment. I don't doubt what you are saying as I am on Linkedin also and have form a couple of groups myself. Are there any stats out there which show lawyers are actually getting a ROI of their time they spend using Linkedin?

* Lawyer profiles on LinkedIn are regularly being looked at by potential and existing clients. Either by people like me who look up everyone at LinkedIn because of the quick easy to scan and complete profiles or by those reaching the profile from a direct link on Google.
* Lawyers are answering questions from reporters and other LinkedIn users as a way to showcase their knowledge and network with potential clients and people who influence them.
* Lawyers are forming groups relevant to their practice area and/or the industry they represent. Results as instant recognition as leader in that niche, including a locale, and is great way to meet others interested in your niche.
* Lawyers blogged content is being displayed in news section of LinkedIn users who have an interest in the services you offer.
No question that lawyers who have developed an understanding of LinkedIn are finding it works well.
Like anything, there are those who question LinkedIn. Some folks think lawyers saying blogs work for marketing have a screw loose. ;)

Yes, thank you. I do not know either, and expressed that recently on my blog with "I Am Not Really Sure I Am Linked In To LinkedIn". So, it accounts for some Big Law finding a cheap way to communicate, like a listserv, or Yahoo Groups, or Basecamp. So what? Like you, I wish someone would enlighten me? Like the old tech bubble days, I tend to think this is a money chasing a story in which there is really no substance.