CVG goes on the record stating "It is said that Microsoft will now showcase its console at a one-off media event in early April - though it is expected that discussion about future tech would be an inevitability if Microsoft engineers and developers attend GDC in March." It is unclear, whether this is actual new information or regurgitation of previous murmurs, as CVG quote an anonymous Sony source as saying their surprise PS4 reveal "ruffled some feathers".
April does make sense if Microsoft wants to have the same kind of pre-E3 showing as Sony did, but it remains to be seen what their approach will be.