My friend Vince deStefano down in Melbourne, AU just sent me a link to his free sky replacement library and his YouTube video tutorial on how to do sky replacement. I'm going to add Vince's library and tutorial to the PFRE blog Resource page.
Vince and his Propertysnaps real estate photography business are a real success story. Vince launched his company in 2006 just about the time I started doing the PFRE blog. Vince has now built his business into a very successful, independent, multiperson full service real estate photography business that besides photography does, twilight photography, elevated 10 pole (a big pole that he built himself) photography, floor plans, aerial photography, copywriting and property websites.
The skyline in Vince's example has a pretty simple skyline. Sometimes you will encounter more complicated skylines with lots of trees. Here is another skyline replacement technique that you can use for the tough skylines. Always give Vince's approach first because it's faster.