Here is a newbie question on corporate supply.

When you know these data about a corporate deal from a newsfeed:
MGM Mirage <MGM.N>:
* Prices $1.5 billion in senior secured notes
* Says priced $650 million in 10.375% senior secured notes due May 2014 at a price of 97.184%
* Says $850 million in 11.125% senior secured notes due November 2017 priced at 97.344%

does it mean that the rate-lock selling already occurred because they priced at a price of ....% ?

rate locking occurs prior to the pricing of a bond. the company can rate lock as little as a day before, to months/years in advance. corporates also don't have to rate lock.

rate locking is not uncommon for high grade issuers. for high yield like mgm, rate locking makes less sense because the sensitivity of the bond coupon is less driven by treasury/swap rates and more on investor demand. in other words, high yield bonds are less sensitive to movements in rates than high grade bonds.