Very interesting point. One that springs to mind immediately is Virtua Fighter 5 for the 360. Recently released here in the UK, last week in October if GameFAQ's anything to go by. A little over 2 months now and what was a full price game for a next-gen title, £50, can be bought online from a GAME store for £20, 60% off and cheaper now than games released for the same console which have been on the market for much longer.
I think I lot of it boils down to what the market is buying. Naturally, game stores will want to keep the popular games at full price, or as close to it as they can, in order to gain maximum profit, but those not selling well (VF5 by the looks of it) they want to reduce the price of in order to shift stock and not keep it laying about otherwise they lose money in keeping it stored. Also depends on the platform it's available on. Certain games are an exception however. ICO for the PS2 was on offer on eBay for ridiculously high prices, but it was largely due to the fact that there weren't that many copies of the game available anyway, making it a worthwhile collectors item.
For the most part, I'm with BarrieV on this, but it largely depends on the game in question and your anticipation of how it will fare in the market. If it's a game such as Halo 3, or Guitar Hero III, you know how popular they are and they will probably be at a high price for somewhile so it's probably for the best to buy it at the cheapest place you can find it. But if it's a relatively obscure JRPG, you know you can afford to wait for a while and the price will be cut. Personally, if I want to buy a game on the release day then it has to be something I've heard good things about, looks fun to play, and I've been waiting a long time for. (FFXII for example)