this is very confusing..
1. you dont need an api key to repost you tube files.
2. even if you did need an api key and you rec'd one from wordpress.com, they wouldnt do the same thing
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Moving on .. why did you sign up at wordpress.com, and what did you get an api key for? askimet? thats the only api key that wordpress.com offers, as far as I know.
that api key is used to enable askimet on a self-hosted personal blog, as explained here:
http://akismet.com/personal/
If youre trying to do something else, im not sure you understand how wordpress.com works.
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This:
http://wordpress.org/support/topic/93034?replies=14
is whats confusing you, I'm guessing and its crap. You do not need any api key to repost you tube vids to blogs.
if you want to do the 'send to blog' thing from within you tube, check this site:
http://www.robertbuzink.nl/journal/2006/11/23/youtube-brackets-wordpress-plugin/
Mind you i do see that he also points you back to wordpress.com for an api key. At any rate follow his directions, see if it works :)