Think it was the gamestation website I was on earlier, for those who pre-ordered before Sunday at midnight (now gone by a good 5 hours) they'd get it by Tuesday, which is release day. Of course, this means it's actually shipped out before it's meant to be released, so I hope you trust your postman cos he might clock it and steal it for themselves to have their wicket way with!
If you DO get it tomorrow (today for me, it's 5:35am on Monday morning, wtf am I still doing up?) then it probably would be frowned upon to the extreme. The biggest game in a decade is, simply put, too big to handle, too big for Take 2 (or Rockstar or whoever) to contain and it leaked out, got shipped early, stolen by the KGB and any other means of obtaining a copy earlier than they wanted!
I have absolutely no idea if people will be allowed to post their clips of GTAIV on YouTube once the game is 'officially' released, I was surprised it was removed the first time around!