I have been wondering the same thing. Although I use iMovie for basic home movie work, I use Adobe Premiere Pro for editing larger projects, id still like to pick up Final Cut Express as my wife has expressed interest in going beyond iMovie as well.
Normally when it comes to Apple and new products, I tell my friends not to wait unless I know for certian something is coming, like in May I will tell friends NOT to buy a new iPhone because we all know in June they get a refresh, be it major or minor. Same thing with the iPods, if in August they are looking for a new iPod for school or university I tell them to hold off until September because, once again Apple always refresh the iPod lineup in September.
As for Final Cut Express 5? I actually am waiting for it, but I don't know if I should, but I know the second I buy the current version Apple will release the update. All I can say is hold off a little while but if you really want it just go for it. As for the rumors of an April or near April release? I have heard nothing of the sort, generally with the Pro or Express software we don't hear much about it until it happens, I mean, look at Aperture 3, it came out of no where.