Out of boredom, I decided to make a spreadsheet looking at the trends of the item level of gear to required level depending on expansion. I have linked the actual graphical representation for those who want to make guesses of what the gear will possibly be like for cataclysm.
I took item levels from greens for each level 1-80, and then epics at each cap level: 60, 70, 80. blues were not consistent.
i left out in between ilvls for epics, and the top top top end ilvl's because they so few and far between. you will notice missing in between numbers, and maybe max ilvl for that cap.
Cool things to see, during pre-bc (vanilla), greens ilvl increased 1 for every level, during bc, the ilvl increased by 3 every level, wotlk ilvl by 4,
also, roughly the top ilvl of epics for that capped level, seems to imply the starting ilvl for greens in the next xpac.
My guess for Cataclysm Expansion based on this, is that starting ilvl for lvl 80 greens will be around 277ish . and that the ilvl will increase by 4-5.
so by lvl 85 green ilvl will be around 299.5 (300 rounding up using 4.5 ilvl per lvl average.)
epics at lvl 85 starting at probably 330 (pulling that out of my butt but eyeing it)