All sarcasm aside, who knows? I don't forsee anything special (as much as we'd all want a 30th anniversarry Mac), but Steve's always got a surprise up his sleeve...
There was a lot of anticipation on these boards about the 20th anniversary of the mac in 2004-- nothing happened; I don't think there was even an apple press release. (The 20th anniversary mac came at the 20th anniversary of the company, not of the mac; for those of you who may not know that). I don't think we'll hear anything about any anniversary.