Ok I think we may have a little bit of a mistake somewhere, because its
+0.4 for every upvote after 2 upvotes, or it was as far as DavidBeck has told me since december 2011 (23/12) through pm's. It used to be +3 for +1 before it was changed at some point during this year.
I.e. this means that
+1 Earns 0 respect
+2 Earns 0 respect
+3 Earns +0.4 respect
+4 Earns +0.8 respect
+5 Earns +1.2 respect
and so on
Let n be the number of upvotes
)
where

It may appear that a +4 post has earned +1 respect if your respect started on xxx.2 (as 0.2+0.8=1.0)
On the stats page it appears that everything is in whole numbers, but that is because the decimal point is dropped.
Just to double check this, me and laseredd have expiremented with upvoting some posts and wacthing the change on the stats page (before when there was decimals and when they have been dropped) and this confirmed the +0.4.
Dw, we unvoted afterwards aswell. Also same goes for downvoting, -0.4 for every downvote after 2 downvotes
EDIT: Added formula.