Oh incidentally, a few here and elsewhere have mentioned mailed invites.
I've not received ANY mailed invites from any of the 3 jarls who sell land.
I did get 2 letters, one from the new owner of the Riften kiddy home and one from Preventius (sp?) the Whiterun steward letting me know I can visit to change my alchemy table for a child's bedroom (yeah right)
For clarity I've been a thane in all the areas for a long time with blades to show for it, got no letters from anyone but Whiterun but still can't buy Falkreath land on one of them but I can on the other.