liquidations will typically begin at approximately 9:40 AM ET

http://ibkb.interactivebrokers.com/node/1718#What_can_I_do_to_prevent_the_assignment_of_a_short_option_

What happens if I am unable to meet the margin requirement on a stock delivery resulting from an option exercise or assignment?
If an option exercise or assignment results in the delivery of a long or short stock position and the account holder does not maintain sufficient equity to meet the ensuing margin requirement, IB will act to liquidate positions to restore margin compliance. While IB retains the right to liquidate at any time in such situations, liquidations involving U.S. security positions will typically begin at approximately 9:40 AM ET as of the business day following expiration.

I fixed the margin issue myself before 9:40am.

Being assigned to 6 short call options, Net $230

Notification of Option Assignment
This communication serves to inform you that an assignment notice has been issued for the short option contracts held in account UXXX238 as listed below.
Contracts:
Option ; Expiration ; Strike ; Type ; Quantity ;
SPY ; 6/2011 ; 125.0 ; CALL ; 6 ;
In accordance with the option assignment procedures, these contracts have been removed from your account and the associated purchase or sales transaction(s) in the underlying asset(s) booked with a trade date of 2011-06-16.
For additional details regarding exercise and assignment transactions in your account, select the Report Management and then Trade Confirmations menu options located in Account Management.
Please contact Customer Service ( IB../p.php ) should you have any questions regarding this matter.
Interactive Brokers Customer Service

Cost of 6 125 call, $1427. After being assigned, don’t need to buy back. 

Realized Profit of 6 123 call, $400.

Realized SPY loss, $1595. 

Net: $230