There's a lot of indebtedness from Wikitravel to Wikipedia:
We use the software developed specifically for Wikipedia to run Wikitravel.
Wikitravel's format, policies, and style were largely inspired by Wikipedia's.
People familiar with Wikipedia are more receptive and familiar with the idea of what we're doing; they blazed the way for collaborative development of reference texts.
Many contributors to Wikitravel are also regular contributors to Wikipedia.
Note, however, that Wikitravel is not a Wikimedia project.
We have a special Welcome, Wikipedians page to introduce our project to people familiar with Wikipedia.
Wikipedia and Wikitravel use incompatible licensing schemes. Wikipedia uses the GNU Free Documentation License ("GFDL"), while Wikitravel uses the Creative Commons Attribution-ShareAlike 1.0 ("by-sa") license (see Why Wikitravel isn't GFDL for more details). Because the GFDL and by-sa aren't compatible, it's not possible to copy content, in most cases, between articles on Wikipedia and articles on Wikitravel.
There are some ways to do it, though. If you are the sole creator of content on Wikipedia, you can move it to Wikitravel; simply note on the talk page that you're relicensing it under Wikitravel's Creative Commons license. Moving content worked on by other people requires getting their permission first; since both projects share the copyleft spirit, this shouldn't be difficult. The process is the same for moving material from Wikitravel to Wikipedia.
However, think carefully before doing this, and discuss it with others. Wikipedia and Wikitravel have distinct goals, and forking large amounts of content from one project to the other creates wasteful duplication. In most cases, linking from one to the other would be a better choice.
Linking to Wikipedia allows people to access encyclopedic information that doesn't fit into the framework of a travel guide. For example, the entry on the United Kingdom should have a thumbnail sketch of the country's history, but detailed information on Henry VIII is inappropriate. A link to Wikipedia's UK article would allow readers to find non-travel related information about their destination of choice, without stretching Wikitravel beyond recognition into an "everything you need to know about everything" guide.
If there's a page on Wikipedia that covers the exact same subject as a page on Wikitravel, but from an encyclopedic rather than travel guide point of view, consider making a twin link to the page. And if you link to Wikipedia, it's also nice to link back from Wikipedia to Wikitravel as well. See links to and from Wikipedia for how to do this. You can also just add regular links to Wikipedia in the text of a Wikitravel article, using Wiki markup.
However, don't use links as an excuse to be lazy. Wikitravel entries should be useful and self-contained; links should provide additional information. Writing "Everything you need to know about France is here, except that there's a good bagel place in Lyon" is not helpful.
There are always various projects in the works at Wikipedia, similiar to Wikitravel's concept of Expeditions. Some of these could be used by both Wikipedia and Wikitravel. See, for example, the work being done on maps.
All Wikimedia wikis use the same software as Wikitravel. The software is called MediaWiki, and it's Free Software under the GNU General Public License (GPL). It's written in PHP, and uses the MySQL database engine to store page and user information. It was originally created by Wikipedia developers, but it's now used for a number of other projects (see sites using MediaWiki for a list).
Helping out with the MediaWiki software means that Wikitravel works better. It also shows that we're grateful for this great software, and that we want to give something back to the people who worked so hard to make it. It builds up goodwill, since anything we do for MediaWiki will help users for all the sites using MediaWiki.
Some things to do to help with MediaWiki:
Report bugs. MediaWiki has a bug tracker; you can use that tool to submit new bugs. Make sure that it's a problem with the software and not with Wikitravel first, of course, and check to make sure that the bug you've identified isn't already in the tracker. But identifying bugs helps developers improve the quality of the software.
Suggest enhancements. If you have a good idea for how to enhance the MediaWiki software, submit a Request For Enhancement ("RFE") on the SourceForge site. Again, try to make sure that the request hasn't already been made in the tracker. Identifying new features for MediaWiki can make it easier for users of all MediaWiki sites to make great content.
Submit patches. If you know PHP, you can review the features and bugs that have been submitted, and try to create patches for them. Patches for MediaWiki should be submitted to the Wikimedia developers mailing list.
Write documentation. There's a Wiki-based, user-edited user's guide for MediaWiki on the Wikipedia meta-site. You can contribute to the user's guide, which will help all users of the software.
Join the MediaWiki project. If you know PHP, you can join the MediaWiki SourceForge project and help with the maintenance of the MediaWiki codebase and its further development.