Advance notice thanks to the Money Saving Expert (MSE) newsletter.
As per title, sale starts 06:00 on Sat 20th Feb for stays 12 March to 31 May 2010
As usual, there will be a limited number of rooms available at this price, and only at 'selected hotels'.
I can't find the details on the site yet but worth knowing about and checking later this week.
EDIT: appears to be live now (19/02/2010)
The "saver_rooms" link now has an advert on it saying
"200,000 rooms at £19 Stay between 12 March - 31 May 2010 Search for rooms below".