To me, the discussion of ubuntu and similar systems being reliable pretty much got settled around two years ago. These systems are reliable, period. It is not about the OS anymore. It is about what people can do with it.
On top of that, yes, you have the economic aspects. They are free, highly compatible, blahblahblah.
Where is gets hairy is now on software support for Linux platforms. Which kind of software does an SME need? (as varied as SME's are there). In this case, you would want to classify SME's according to their industry and then actually see if there is software that can competitively run on Linux. Some examples:
Graphic design/advertising: Where is the Adobre Creative Suite? Will an SME use GIMP/Inkskape/Scribus instead of CS5? I doubt it.
Architecture/Engineering: What about AutoCAD, SolidWorks, etc?
GIS and Mapping: We nail this one. Open Source GIS in plain awesome. It is not up to the standard of ArcGIS, but it can pay up if you put in on a cost-benefit analysis.
Music/Video/Media? I think Open source holds up ok. Accounting/Business management: Some good software out there. Can do the job.
Research SME's: yes... lots of modelling, statistics a scientific software out there. I would bite.
Web development: no idea... feedback anyone?