Well, give up looking for a "beautiful blue sky" filter or some other two-click method of doing what you want, at least if you want it to look natural.
"WHAT!" you say. "I've spent all that money on CS-5 and it can't even figure out how to make a washed out sky blue?" ANSWER: Unfortunately, that's right.
Photoshop CS-5 is an amazing program, but changing skies is actually a very difficult thing to do right and in many cases will challenge even the most experienced Photoshop user. Selecting the sky accurately, so that you isolate it from the rest of the image, is the first problem. Entire books are written on the subject of how to make good selections. CS5 makes it easier, but it still requires some operator skill and that comes through study and practice.
Once you have the sky isolated, you can easily sample the color from one image and fill the second sky with that color, but it will look terrible. The most perfect blue sky is not an even blue, so unless you're drawing kids cartoon books, simply filling the sky with a nice blue color isn't going to cut it. Useful tricks for this sort of stuff include using different blending modes, layer masking, adding noise, etc. and each one of these skills have been the subject of countless books, tutorials, and videos. You're not going to learn everything you need to know in this thread, that's for sure. Suffice it to say, doing it so that it doesn't look fake is not as easy as we would all wish.
Having said that, I sometimes surprise myself when using ACR if a pale blue sky needs a little boost. Using the saturation part of the HSL panel to put an adjustment point on the sky and then using the scrubby slider to improve the color often produces surprisingly good results. It works the same in LR too.
It certainly is easy and I find myself using it on many of my snapshots, but it's not quite the same thing you're after.
Bottom line: It ain't all that easy to do it correctly.
