Billing
I searched high and low, but was never able to find a satisfactory time-tracking and billing solution under Linux. I finally ended up installing Windows XP in a VirtualBox so I could run PCLaw. It works great, but it's far from ideal to install two operating systems just to run a billing program.
Practice Management
Again, a thorough search failed to reveal any law office practice management software that runs under GNU/Linux. Many people use Clio or other cloud-based systems, which are completely platform-independent, but if you want software that runs locally on your machine, you're out of luck unless you want to run Windows in a VirtualBox.
Word Processing
Neither Microsoft Word nor WordPerfect run natively under GNU/Linux. Fortunately, both programs run under wine, which is essentially a Windows emulator that allows many Windows programs to run seamlessly in Linux. Under wine Windows and Linux programs can share the same clipboard, printers, and files. The problem is that while wine has come a long way and is a very usable product, it is far from perfect. For example, I occasionally have problems printing from both Word and WordPerfect. While this is not a major issue for me, as I can always find a workaround, it would be a headache attempting to support an entire law firm running Word and/or WordPerfect on Linux.
OpenOffice (or its new cousin LibreOffice) is a great, fully-functional office suite that could serve as a free replacement for Microsoft Office or WordPerfect Office, except for one problem: compatibility with Microsoft Word. OpenOffice does a poor job of reading and writing complex Word files, especially pleadings, documents with fancy letterheads, and documents with headers and/or footers. OpenOffice is great for internal documents, but it is simply not practical when we as lawyers are constantly exchanging Word documents with our clients.
OCR/PDF
There are still no solid OCR products for GNU/Linux, but OmniPage does run almost perfectly under wine. The problem lies more with PDF editing, which is essential for document review and redaction. While Acrobat pretends to be multi-platform, Adobe has only ever made its Acrobat Reader available under Linux (not the Standard or Professional versions of Acrobat). I use Acrobat 5.0, which runs almost perfectly under wine, but is now very out of date.
Usability
GNU/Linux has come a long, long way in terms of ease of use, thanks in large part to Ubuntu and its community. But it still has a ways to go before I would be comfortable recommending it on a law office desktop. Just to give a few examples, my Java support broke a few months ago when Canonical (the company that distributes Ubuntu) decided to "upgrade" my Sun Java by replacing it with non-working software due to a licensing issue. That just is not acceptable for a business OS. Similarly, Flash stopped working in my Chrome browser a few months ago because Google (the maker of Chrome) failed to update the plugin for some reason. Those sorts of things don't happen very often under Linux, but it's often enough to indicate that it's not yet a rock-solid business environment. I was able to fix both problems after doing some Googling, but I probably wasted about two hours of time.
Conclusion
It is certainly possible to run a law office on Linux, as I've been doing for over 2 years, but it is not yet optimal to do so. The lack of applications that lawyers need requires resorting to wine and/or VirtualBox, and there are problems and inconveniences associated with both.