Wryxian has written a book on fishing over on the forums -- he originally responded to the thread with a quick link that affirmed in patch 3.3, you will never ever catch trash from fishing pools, regardless of your fishing level. If you put the bobber in a pool, you'll get what you're supposed to, no matter what (presumably to allow even low level players to do the fishing quests). But players have problems with that. First, there's the old issue of things always getting easier as the game changes -- people are complaining that they had to level with missed catches and trash, and those who level fishing after 3.3 won't. For that, I don't have a better answer than Wryxian does: that's just the way it works. The game is ever-changing, and most changes make things easier. That shouldn't devalue what you did before.
The other issue is that this may destroy the fishing economy -- if even low level characters always get the fish they want 100% of the time, that means more fish in the market, which means any money high-level fishermen would have made off of rare fish will probably go bye-bye. But Wryxian says that's OK -- people will still use fish for themselves, and he believes that the pain of losing that AH sale will be solved by the bonus of always getting the fish you want. We'll have to see -- it's true that between the updates to cooking lately (and the future fishy updates we're expecting), Blizzard is making sure the secondary professions are much less exclusive, which means there has to be the balance of also making them less lucrative.
Patch 3.3 is the last major patch of Wrath of the Lich King. With the new Icecrown Citadel 5-man dungeons and 10/25-man raid arriving soon, patch 3.3 will deal the final blow to Arthas. WoW.com's Guide to Patch 3.3 will keep you updated with all the latest patch news.