You can visit us at Twilio anytime! Email me danielle@twilio.com and I can set it up so that we have time to hang out with some of the team for a meal or something.
SAN FRANCISCO
One place to meet tons of awesome people: Pier 38 / Dogpatch Labs is home to at least a dozen tech companies including Threadsy, Cotweet, 99 Designs, and a bunch of stealth ones, too. You could also try Citizen Space and Parisoma, they're awesome co-working spots with several startups each.
SEATTLE
The Seattle 2.0 Startup Index might be a good place to start there: http://www.seattle20.com/startup-index.aspx
There is a concentration of startups in 3 neighborhoods: Pioneer Square (by the ballpark and Amazon), Capital Hill (the "hip" part of town), and East Lake Union (up and coming, near new Amazon campus)
Shortlist of startups I'd hit up: TeachStreet, Cheezburger Network, SEOmoz, Kashless, Gist, GarageBand.com (shares office with iLike/mySpace), many more since I left there