SQL CASE WHEN IN WHERE CLAUSE

I have following Scenario

Something has got expirydate and i need to implement following conditons

SELECT * FROM SOMETABLE

WHERE EXPIRYDATE { should based on following conditons}

1. Should > 2012-12-31

2. Should be between GETDATE()+90 , IF Process RUNS after 2012-12-31 or 2012-12-31 + 90 days if process runs before 2012-12-31

Can you help me form this

Saadat