Typically freezing something living kills it. The reason is that ice crystals break the cell walls that are critical for life.
Back in undergrad, I did quite a bit of research in a biochem lab. We didn't work with yeast, but we did use quite a lot of E. coli containing various genes we wanted to express as proteins. In the lab, you often need to store bacteria for long periods of time as a way of storing bits of DNA sequence or a plasmid you want to use again.
I figured that the method we used to store bacteria was probably applicable to the storage of yeast.
The way we did it was to first concentrate the cells by centrifuging. Then we would re-suspend the cells in a solution of buffer (neutral pH salt solution) and glycerol. We could then store the cells at -80 degrees centigrade for several years.
I tried a similar approach to storing yeast for home brewing. I kept about 10ml of a conical tube of White Labs California Ale yeast, using the remainder for a pale ale. To the remainder I added some sugar water boiled with a few teaspoons of chicken broth (yeast nutrient would be better, I just wanted to supply some nitrogenous compounds, amino acids that are not present in sugar water). I let it sit for a couple days on the counter. I released the pressure in the tube and added about 20% glycerol (eyeballed it). You can get glycerol at almost any pharmacy. Threw the tube the in freezer (just a regular freezer, about -20 degrees C).
Now about two months later, I pulled the tube out and grew up a starter with some extra LME. The yeast grew up well. I put in the fridge to precipitate as much of the yeast as possible out of solution, decanted the supernatant, and then pitched it into my ESB wort. 24 hours later, I have strong air lock activity.
Just an interesting alternative way of preserving microorganisms that I hadn't previously seen mentioned here. Fairly simple to do. Glycerol protects the yeast cells from freezing temperatures by preventing ice crystal formation. The tube stays liquid the entire time.