Weighing popularity

I'm a little stuck on this. I have a table like this:

posts (
id integer,
item_id integer,
linked_on datetime
)

where item_id is a reference to another table and many items will show
up multiple times over an amount of time. I'd like to display the
posts table based on the weight of how many times an item shows up
over x number of hours.

Data might look something like this (assuming it's 2007-04-15 11:30):
1, 3, 2007-04-14 23:20
2, 1, 2007-04-15 07:00
3, 1, 2007-04-15 08:30
4, 2, 2007-04-15 11:00
5, 1, 2007-04-15 11:10
6, 2, 2007-04-15 11:12

so, in this case item #2 would bubble up to the top if show popularity
over the last hour (followed by item #1 then item #3), but item #1
would bubble to the top if looking at popularity over the last 6 hours
(followed by item #2 then item #3).

Thanks for the help!