I was poking around in a thread on the Consortium's forums about how best to jump start the leveling process for maximum efficiency after the Cataclysm launch. In reading the various strategies, it occurred to me that there hasn't been a true head-to-head comparison of the different ways which we'll be able to get XP in the future. So I took it upon myself to collect some numbers.
For those planning to stockpile a bunch of WotLK quests for instant XP increases, here's the numbers from some current dailies on a level 80 character:
Fishing Daily in Dalaran = 23152 xp
Cooking Daily in Dalaran = 17377 xp
And here are some numbers for the new content we'll be getting with Cataclysm:
New Cooking Daily in Stormwind = 23152 xp
New Fishing Daily in Stormwind = 23152 xp
Level 80 mob killed = 4556 xp (2278 rested)
Level 80 quest = 29084 xp
And just for comparison purposes, here are the current XP values for crafting-related activities:
Level 80 herb = 4620 (Cinderbloom/Stormvine/Azshara’s Veil)
Level 80 ore = currently broken but presumed the same
Archaeology Fragment Looted = 23204xp
Full Dig Site Clear = 69612xp (3x fragments looted per site)
You can see that the XP values for gathering are equivalent to killing mobs, at least in the current Beta build. The amount of XP for collecting Archaeology fragments is actually quite high compared to quest XP. I'm sure part of the reason for this is the large amount of travel involved in Archaeology which doesn't apply nearly as much to questing.
Naturally these numbers could change at any point, but I think they make a decent argument for including at least your gathering professions and maybe even some light Archaeology into your leveling plans.