I found out this week that I will being reassigned to Fort Lewis in January. Where are the best places to ride? Are there any good tri clubs? Best bike shops? Anything else I should be aware of…
I grew up in the area. Pt. Defiance is an awesome park with a great 5 mile road cycling loop and at least 20mi of beautiful trails to run on. Riding across the Tacoma Narrows Bridge will let you hook up with the Cushman trail bike path which will lead you into Gig Harbor and lots of great, low-traffic riding from there out on the peninsula.
There’s a triathlon series at Ft. Lewis and some great lakes for open water swimming. The weather is beautiful for 6 months of the year and drizzly/gray for the other 6 months.
Where exactly will you be living? On the base or in town (and what town)?
Check out South Sound Triathletes… very active local club located adjacent to JBLM. Great rides. Once you learn the area you’ll find that you can ride from the base to the north end of Tacoma (Pt Defiance), across the Narrows Bridge over the the Kitsap/Olympic Pennisula, or out to Mt Rainer.
Right now I’m leaning toward living on post. The few times I’ve been out there the traffic was insane.
Traffic sucks and it rains a lot. Don’t be the guy with the umbrella, everyone will think you’re a tourist.
In January? Make sure you arrive with a trainer, fenders, breathable raingear, and quality lighting. It’s going to be wet and dark…unless things flip back to La Nina quick. A previous poster said to expect 6 months of great weather, realistically it’s more like 3 months of nice weather, 4 months of transitional weather, and 5 months of rain.
Other notables: Mountain biking in Capital Forest near Olympia and at Dash Point State Park, countless epic trail runs within an hour’s drive, and classic climbs on the road around Rainier. Check out buduracing.com as a starting point for races.
+1 to Budu racing as a good starting point for races cheap, local races
+1 to trail running/MTB
+1 to fenders, lights, & rain gear… ask more specific questions if you haven’t spend time in this climate - the rain isn’t hard, but it is a steady mist/drizzle for a few months. different than Tstorms, different than fog.
other highlights:
great mexican food (a rarity in the pacNW)
and you are within easy striking distance of a good city, (my bias, does it show?) 
I work on base so I know the area pretty well. You are probably better off living on post to avoid the traffic. It will give you much more time to train and easy access to multiple pools and great places to run. The roads around the impact areas on Fort Lewis are perfect for doing long, uninterrupted intervals. There are tons of great places to ride in the area including Yelm/Tenino, Steilacoom, Gig Harbor, Browns Point, Orting, Enumclaw, the Green River valley and even in Tacoma itself. I am not a triathlete so I can’t speak to the local clubs but the best bike shop in the area is easily Tacoma Bike. Tony Corke is their bike fitter and is head and shoulders above anyone else in the area. If you race road bikes I can give you a bunch more info as well or you can PM me.
+1 on Tacoma Bike 
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One of my first thoughts when I found out I was moving out there was that its time to get a mountain bike. What’s better for the terrain, a hard tail or suspension?
No, I’ve never spent any significant time in the area or that type of climate. Arctic, desert, tropics, yes. I’ve spent 4 months over 2 summers there and both times I’ve been out there it rained only once. However, after the first week of sleeping outside I realized that I needed much warmer clothes. A person shouldn’t freeze their ass off in July when being outside.
Thanks for all of the information everyone.
http://www.southsoundtriathletes.com/
http://sports.groups.yahoo.com/group/SouthSoundTriathletes/
They work out of/with Tacoma bike. They have weekly rides on base as well as other events.
I haven’t met up with them yet because of scheduling issues.
MArk B.