The 19 April date looks pretty firm for bmi to leave Star Alliance. Lufthansa has added a web page today

With effect from 19 April 2012, Miles & More’s cooperation with bmi will end by mutual agreement. This is the result of the sale of bmi to IAG and the consequent withdrawal of bmi from the Lufthansa Group.

As of that date earning miles on bmi flights will unfortunately no longer be possible. Until then the current terms shall apply.

Miles & More flight awards issued prior to this date can be used as planned on bmi flights.

Status-related benefits on bmi flights can still be used until 31 May 2012.

Quite what ‘Status-related benefits …. can still be used until 31 May 2012’ means is not clear, but could mean that LH Gold Star Alliance cards will get you in to bmi lounges until 31 May. However, it bodes badly for me being able to get my bmi flights on 20 April in to United.