You can easily import WordPress to Tumblr. It took me a while to find an easy solution. I found some guys posting scripts to download, but they seemed potentially out of date, dangerous and complicated.
Tumblr doesn't provide a way to import from WordPress; it's not part of their missions, but open source WordPress does have a plugin that supports exporting a WordPress blog into a Tumblr Compatible backup file.
This plugin is compatible with WordPress version 3.0.5 or higher. The plugin doesn't have a rating yet, so use at your own risk.
I would advise against importing your WordPress blog to Tumblr. WordPress is open source so it has more options and is quicker to evolve with new technologies. This is partly what makes WordPress better than Tumblr. Furthermore, with a self hosted WordPress website you have the option to host wherever you want. If your site gets big, you can upgrade you WordPress website to a dedicated server.