I recently grew up from a 17-month old slurry of 2206. The starters and the main batch behaved like I would expect. I think if you're conservative about your viability estimates (and have good sanitation, obviously) you can keep yeast in the fridge indefinitely. The viable cell count will get very low after a while, but I don't know of any reason that would affect the performance of the yeast that do survive.
Maybe Palmer meant that six months was the maximum for direct pitching? If so I wouldn't even go that long.
Here's what Palmer says...
"If you harvest yeast from the primary fermentor, you will need to separate the yeast from all the trub that is mixed in. Professional brewers most often do this by "acid washing" the yeast--using acid to lower the pH to about 2.5 so that bacteria is inhibited and using whirlpool methods to seperate the heavier trub from the lighter yeast. But acid washing tends to inhibit the yeast too, and is not strictly necessary. You can simply use chilled boiled* water and two sanitized jars to separate the healthy yeast (white) away from the majority of the trub.
After racking the beer, swirl up the yeast layer on the bottom and pour some into a large sanitized jar (such as a mayonnaise jar).
Gently pour in some cold, boiled water and swirl it up to get all the yeast and trub in suspension.
Let the jar sit for a minute or three to allow most of the trub to settle to the bottom. Gently pour the cloudy water, containing suspended yeast, into another sanitized jar. Discard the dark trub.
Add some more water and repeat this procedure until you are left with a substantially light-colored yeast suspension and only a thin brown layer of dead yeast and trub on the bottom of the jar.
Store the jar in the refridgerator for up to a couple months. The yeast will turn brown as it ages. Discard it once it turns the color of peanut butter. Eventually the yeast will autolyze and die as its nutritional reserves are used up.
Pitch the yeast to a starter before using to ensure its vitality. If the starter smells wrong--rancid, vinegary, etc., the yeast may be contaminated. The dominant smell of a starter should be a yeasty smell, but sulfur smells are not necessarily bad, especially with lager yeast strains."http://www.howtobrew.com/section1/chapter6-8.html
17 month old yeast...wow....I'm pleasantly surprised. That's good to know.