I think the simplest case will the GNSx30 boxes, which all do 8.33. There is a setup change in one of the user-accessible setup pages which enables this. The result is more knob twiddling...
Another reasonably easy one will be owners of KX155A radios, which were commonly installed after about 2000. You can plug in a KX165A. If the 155A was driving a CDI or HSI then the 165A may need to be correctly configured but apart from that it's a plug-in change. The cost is significant however - example.
Those who can't do the above will have to install something which perhaps needs a bit of rewiring. There are various options, starting from around the €1k mark like this which are little round radios, to the standard-size stuff, and Garmin are offering some of that stuff (of course - $$$$).
This is really going to upset the VFR-only community, who will get zero benefit because probably nobody will be allocating 8.33 frequencies to stations which they might be talking to.
However one can't be sure, because my recollection of the crazy UK OFCOM frequency tax proposal is that they were going to charge less tax for 8.33kHz frequencies than for 25kHz ones, and that would push a lot of GA airfields to change to 8.33. Does anybody know how far that proposal got? I can't see anything up to date on a quick google.
The other curious angle is how many 8.33 radios will be needed. Last I heard was that it depends on whose airspace you are flying in, and in the UK the requirement was for just 1.
The whole thing is stupid because the entire need for 8.33 arises only because each European country wants to keep its frequency allocation office. If allocation was done centrally, 25kHz would be fine - just like it is in the USA.
It will be worth having a good look around the Friedrichshafen show in April, because everybody and their dog who can make a VHF radio will be looking to make a fast buck out of this 
And my poor self, not legally bound to carry any radio at all, might well find some nice 2nd hand non 8,33 set shortly.
I'm thinking this will be helpful for buying a 2nd COM box for my aircraft as I already have GNS430 for COM1 expect prices will fall for non 8.3 :-)
I've also thought some streamlining of frequencies could be accomplished. If we take an airport like Doncaster it has something like 20 movements a day (and that's a good day) of which say 6 are CAT yet it has the following frequencies.
Ground, Tower, Approach, ATIS, Fire,
Are all of these really necessary for such a low number of movements?
It doesn't seem that long since we had to get the right crystals for a specific trip. It was 100KHz channeling then and we didn't carry all of them!
It is an important gotcha to be aware of. I had a bit of a barney last year when Deauville handed me to an 8.33 London Control frequency at about FL80 and I had forgotten how to activate it on the 430. After a merry-go-round of "Negative 8.33", I eventually got the usual London Frequency and was rather bemused by the whole business.
The upshot of course is that I now have 8.33 set on the whole time, and those dastardly Frenchies won't catch me out again.
Are there any disadvantage or gotcha's to leaving 8.33 set on the whole time.
Only that the knob wears out faster
Are there any disadvantage or gotcha's to leaving 8.33 set on the whole time.
If you use the knob (not mandatory on GTN650/750s), it takes longer to scroll through to the frequencies.
And your wrist! Really, it is not to be underestimated. At least on the GNS430/530, whose knobs have a fairly high "resistance", it can be a real pain (literally!)...
