Hi. When you buy a domain name and use the domain name mapping upgrade for your WordPress.COM blog, everything is exactly the same, the only difference is that instead of being teknophilia.wordpress.com/blablabla, it would be yourdomain.com/blablabla. More info here:
If the domain name you wanted has already been taken but uses a registered trademark that you own, get professional legal advice. Otherwise, I doubt you have any recourses. You could always settle for something similar (for example, if .com is registered, try .net) or very worst case scenario, make the current owner an offer...
Moral of the story: if the domain you want has not yet been purchased, go grab it before someone else does.