Tonight's results:

Arsenal 3 - 1 Hamburg
CSKA    0 - 2 Porto

This means that...

if we beat Porto in the final game...

we will go through as group winners and CSKA can go through at Porto's expense if they beat Hamburg.

if we lose to Porto...

if CSKA beat Hamburg we will go out (and into the UEFA cup as 3rd place in the group)...
otherwise we go through in second place (Porto win the group)

if we draw with Porto...

we go through as group winners. This is because
1) we beat Porto on the head-to-head (which comes into play if CSKA don't beat Hamburg)
2) if CSKA do beat Hamburg, all 3 teams would be on 11 points, and Arsenal win the mini-league

       Pld W D L F A Pts
CSKA    4  1 2 1 1 2  5
Arsenal 3  1 1 1 2 1  4
Porto   3  1 1 1 2 2  4

The Arsenal-Porto draw would take us both to 5 points, without changing the goal differences.

So, a draw in that game takes both teams through. Which can only be a good thing. Porto would prefer to win and top the group, of course, but if it's tied near the end (and CSKA are ahead) neither team may want to risk going out.

For competition regulations, see UEFA's pdf file (rule 4.05 explains what happens when teams are level on points).

While we're at it, congratulations to Celtic whose win over Man Utd tonight takes them through to the group stage and leaves United needing a point from their last game, at home to Benfica.