Packing around a bike lock on a bicycle tour is not something I particularly like to do, but as much as I like to put my trust in strangers, there are few worse feelings than walking out of a restaurant to see an empty space where your bike should be.
There are a lot of compact and lightweight bike locks, such as the Knog Milkman, that do a good job of keeping honest people honest with a minimalist cable. If you're like me and also want to keep dishonest people looking elsewhere, a sturdy U-lock is often the way to go. Unfortunately these are almost always heavy and cumbersome.
Tallac Design has recently taken a fresh look at the U-lock design with their new Vier lock. This is a packable lock that breaks down into four pieces and fits into its own small carrying pouch that you can toss in your back jersey pocket. The zippered carrying pouch also has a strap that allows you to secure it under your bike seat. This gives you the urban security of a U-lock, with a compact design a bicycle traveler can appreciate.
The current design provides about 5" of length to lock your bike to a rack, however, since this can be disassembled, there are plans to offer longer shackle lengths once the lock gets off the ground. It is currently in the crowdsourcing stage on Kickstarter, and last time I checked it was halfway to meeting its fundraising goal. If you're into this concept, I urge you to throw a couple bucks their way and help them put it into production.