Is it legal to use bitcoin in a domain name? Is it legal to use bitcoin in a domain name? Not a sub domain, but in the main domain name.
To date, no entity has trademarked word "Bitcoin" in the US. This means that it's highly unlikely that any US entity would file against your domains under the ICANN Uniform Domain-Name Dispute-Resolution Policy.

There was a servicemark application for Bitcoin, but it was abandoned in July 2011.

Related trademark applications


Bitpay applied for Merchant Solutions for the bitcoin currency and Bitcoin Merchant Solutions, both also abandoned.
There is one for Bitcoin appliance, also abandoned.
Bitcoin Magazine applied for a trademark, and it's still in processing.