Based on the Curse beta database (
http://wow.cursebeta.com/database/items/), items in TBC scale like this (iLvl = Item Level, rLvl = Required Level):
rLvl iLvl
Green Blue
57 81
58 84
59 87
60 90 85
61 93 88
62 96 91
63 99 94
64 102 97
65 105 100
66 108 103
67 111 106
68 114 109
69 117 112
70 120 115
So for uncommon items: iLvl = 3*rLvl - 90
And for rare items: iLvl = 3*rLvl - 95
There are quite a few items that don't fit this scaling though. And there aren't enough epic items in the level 61-69 range yet in the database to say anything about them. Epic items with rLvl 70 generally have iLvl of 100, 110 or 115.
Armor values seem to scale normally with item level, with one significant change: Some, if not most, uncommon items have only 95% of the armor value of what would be expected. There are uncommon items with the expected armor value too though.
Weapon DPS values seem lower across the board compared to iLvl.
Item stat budgets are a bit trickier since there's a lot to consider, like the ratings. From a quick look the amount of stats seems to vary quite wildly though.