These are some copyright-related issues that Wikitravellers should know about incorporating other people's work into Wikitravel. Note that most Wikitravellers are not lawyers; those that are probably practice in a country or jurisdiction far from you. If you need legal advice, contact a lawyer near you.
Information vs. representation
In general, it's not possible to copyright information itself -- just the expression of that information. For example, someone can copyright the statement "The Foofaraw Hotel, at 117 Blurbaw Street in the Carabiner District, has rooms for $50 to $100 including breakfast and in-room television." But the essential facts in that sentence -- that a hotel named Foofaraw exists, where it's located, and what its prices are -- can be used by anyone. Even a false or fictious fact cannot be copyrighted.
Wikitravel can contain information about hotels, restaurants, bars, and sights that are in other travel guides already. However, we need to express that information differently, and not just copy it out of a book somewhere. It's best to get the information first-hand to make sure it's true and reliable; at the least we should fact-check the information in other guides.
In addition, selection and organization of data can be copyrighted, too. So, listing the same 18 hotels in a city that another travel guide lists will be at least suspicious, if not cause for alarm. In general, we should avoid leaning on other travel guides for their information.
The public domain
Certain expression is not copyrighted by anyone; this information is in the public domain. Anyone can use it, copy it, or incorporate it into copyrighted works. In general, works are in the public domain if they're so old that the terms of their copyright has lapsed, or if they were created by government employees or officials (depending on the jurisdiction).
Some public domain works are useful for incorporation into Wikitravel, but most are not. A travel guidebook from 1894 is not going to have all that much information relevant for today's travellers, and will probably be in an offputting archaic style. Information put out by government officials will probably also not be terribly useful.
There is a principle in American copyright law, and in laws of other countries, that allows incorporation of copyrighted works into other works. This principle is called fair use; among other things, it allows adding short quotes and excerpts from other works into our own.
However, it's important to remember that fair use is contingent on the type of person or party reusing a work, and their intention. It may in fact be "fair use" for us to incorporate a picture, text, or other copyrighted expression into Wikitravel for use on wikitravel.org, but it would not be fair for readers to use that same picture in works they derive from Wikitravel. The great benefit of having a free travel guide is lost if people aren't really free to use and reuse our articles.
Therefore, fair use in Wikitravel of works from other sources, with the exception of short quotes or excerpts of text, is not acceptable.