My dad is a lawyer, albeit not in the US, but in Europe, but I think it would still somewhat be applicable.
I tried to switch him to Linux - mainly because I administer his computer and Linux is what I know (I have used Linux on desktop exclusively since GTK-1, I genuinely don't know what how to deal with Windows). This was around 2006 and these are his needs and the obstacles I faced:
1) A word processor: This is the most important part of his business. I switched him easily to OpenOffice in terms of him learning the new environment. However, the problem arose when he needed to email his documents to his clients or colleagues. There were always compatibility issues when saving documents for Microsoft Word. First of all, OpenOffice could never properly deal with headers on pages - I set up his default template to use a header only on the first page (with a letterhead), but whenever he exported the document to MS format, the header would appear on all pages. This was a major pain in the ass (and the last time I checked, the bug was still not fixed). Other compatibility issues weren't that big of a problem, but still annoying to both him and me.
2) Software for accessing laws and precedents: Several companies in our country offer this software, but it's always Windows only. There was an option to run it through Wine, but he would then lose the support (and this piece of software is essential to be working and up-to-date), so I didn't pursue it.
3) Organization of documents: He doesn't use a particular piece of software for this, but rather sorts his documents in different directories. Works for him. This is actually where Linux was miles ahead of Windows. GNOME's Nautilus is actually very easy to manipulate, and at the time the concept of desktop search was growing. With Beagle & deskbar applet, his documentation manipulation got VERY easy.
4) E-mail was not a big deal. Best email clients are cross-platform anyway.
5) Every once in a while he would receive things which absolutely required Windows. E.g. some forensic images which could be opened only with a certain program, or documents with custom made encryption which require a windows software to decrypt, or even USB sticks that need a Windows machine to open them. At the time, a spare Windows machine was used for this, and now, a virtualized Windows would be utilized.
Mainly because of the special software he needed and shortcomings of OpenOffice, we decided to abandon the whole Linux migration process. He is now running Windows 7 and is very happy with it.
I think his next computer will likely be a Mac. OS X runs MS Office happily (and Apple's own office suite seems very nice too), and with virtualization, we will be able to deal with problematic software.