Moving from Drupal to Wordpress
Migrating from Drupal/Civi to Wordpress/Civi is relatively painless.  You'll need to truncate your civicrm_uf_match database table, which links Civi records to CMS users, and repopulate it - Administer menu > Users and Settings > Sync Users to Contacts works well if you want ALL your Wordpress users to be linked up.

That said, there are numerous gotchas to watch out for - I've started compiling a list here.
UPDATE: I've merged this with documentation on wiki.civicrm.org.  Please check Moving from Drupal to Wordpress.

Since StackExchange prefers direct text to links, here's what I've got so far:


PayPal IPNs needed changing. You can only change future IPNs in Paypal; you may also need to set up a 307 redirect from the old URL.
URLs linking to contribution pages etc. in mailing templates, scheduled reminders etc. need to be changed.
Images in mailing templates and scheduled reminders must get a new absolute URL (or a redirect).