The OLPC laptop has a unique feature, which up until now, only Apple has possessed. The hardware and software configurations are known. So this means that the people writing the software and drivers for the OLPC laptops know exactly what they are writing for.
Up until now, every version of Linux has suffered from this problem. Yes, Ubuntu and the newer flavours of Linux are far better at recognising hardware, and choosing the correct drivers, but soundcards and USB peripherals still often cause problems. OLPC won't have this issue.
When wrestling with a new distribution of Linux, I've often wondered why developers don't simply post known-working images of their distributions for each new type of hardware, so for example, if you're using a Dell Latitude D600, there would be a specific image for you.
Over time, experienced users would create these images, and upload them back to the organisers for validation. This would make the installation process much simpler in my opinion, but perhaps the problem is bandwidth. So the OLPC project could create an open-source, Linux based PC that "just works".
In other words, OLPC could erode one of the unique points of owning a Mac. Thankfully, the manufacturers don't have to worry as the OLPC leadership have chosen not to compete with the commercial laptop market, but it does beg the question - at what point of market saturation does the OLPC project become a problem for Microsoft and Apple?
If the OLPC project gains the same sort of momentum as other open-source projects such as firefox & wikipedia, we could finally see a flavour of Linux which works flawlessly straight out of the box on millions of (bright green) PCs.