Hi Steven,

In comparing your request with that defined in the request token fetch stage (http://developer.yahoo.com/oauth/guide/oauth-requesttoken.html) what I can see off the bat is that you're missing the oauth_callback parameter, which should match the application URL which you used to get the application key. The xoauth_lang_pref is optional, so there's no need to include that. Here's the dump of a sample request:

https://api.login.yahoo.com/oauth/v2/
get_request_token?oauth_nonce=ce2130523f788f313f76314ed3965ea6
&oauth_timestamp=1202956957
&oauth_consumer_key=123456891011121314151617181920
&oauth_signature_method=plaintext
&oauth_signature=abcdef
&oauth_version=1.0
&xoauth_lang_pref="en-us"
&oauth_callback="http://yoursite.com/callback"

- Jon

Yet it doesn't work, as for me.
I just fill the correct values and still get 401 Unauthorized.
If I enter the URL directly in the browser I get Custom port is not allowed or the host is not registered with this consumer key.