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).