About Italy, it depends very much where you are going. Big cities can be very hot and sultry during summer, but you'll get the advantage that most italians takes their vacation during July and (above all) August, so cities are mostly empty and easier to tour.
If you plan a beach vacation, it will be much more pleasurable, but you'll have to struggle with peak season, so a lot of people on the beach and high prices.
Weather can be very variable during summer. For example in 2011 we had very hot June and September and more temperate temperature during July and August. However it will never be like Orlando. I went to WDW in September and I've never been in such an humid and hot place before than. That's the reason I'll come in January next time. 13 days to go


About air conditioning: it is getting more and more popular every year. You'll find that a lot of restaurants have it (above all in the most turistic places) and almost all the higher ranked hotels. If you look for a budget hotel, instead, you should check before if it has air conditioning.
In Rome most the Buses have air conditioning now and they are changing metro trains too (about 50% of those have it). I don't know about other cities.
I live in rome, so if you need other informations about Italy and/or my city please ask