Argh I've got this message today, also on BA98. I don't normally have a problem with showing up 1.5 hours before the new scheduled time but I haven't ever seen this message before. I hate waiting around in airports when I have no lounge access (ie when BA or a FT member haven't taken pity on me).
Delay seems to be because the inbound is expected late. It took off on time but I guess they are expecting winds to slow the plane down - these things can change in flight though so I don't think I'll risk it.