I've just set up ARC2 and the white line it generates (that's the fixed curve right?) is fairly flat but the is still a -3db dip at about 75hz.
 
What does it mean when there is still a dip in the corrected curve - is that frequency still going to be a problem?  Why doesn't it flatten it out there too?

Yes,the white line is the corrections (after).
The Gold line is where your frequencies were before correction.
The green is actually the target (flat) Ideal that Arc is shooting for , so the white line dip is just to try to get it to the target (green) flat. 
I usually shoot the room twice when setting up, takes a little time but worth it.
By the way are you following the diagram for shooting, that closely represents your room setup?
 

I have a related question (one I've never been able to properly figure out) - where do you put you when taking the measurements?  The mic is often where my head would be, so I usually duck below my desk when the measurements take place which kinda removes me from the audio picture.  I'm not sure if that's the best thing to do or not..?

ston
I have a related question (one I've never been able to properly figure out) - where do you put you when taking the measurements?  The mic is often where my head would be, so I usually duck below my desk when the measurements take place which kinda removes me from the audio picture.  I'm not sure if that's the best thing to do or not..?

I have found the best results come from stepping out of the room completely.
It gives you enough time before the test tones start for you to move away.

ston
I have a related question (one I've never been able to properly figure out) - where do you put you when taking the measurements?  The mic is often where my head would be, so I usually duck below my desk when the measurements take place which kinda removes me from the audio picture.  I'm not sure if that's the best thing to do or not..?

I have found the best results come from stepping out of the room completely.
It gives you enough time before the test tones start for you to move away.

Ta.  I guess your bod will have a slight damping effect then when mixing etc. which probably isn't a bad thing.

I step back a few feet behind for each measurement.
 
I love ARC2, BTW.  I did an 11 point setup for it at and around the sweet spot.  It's like taking a wet blanket off of my monitors.
 
 

THERE are some room anomalies that the program just cannot solve, 
and it also reads the limit of your monitors, and will not correct past there,
but it is possible to disable the automatic roll-off point detection by selecting Full Bass Range Correction mode.

I'd not noticed that before; it was on by default for me, but the amount of difference it makes for my setup is minimal, especially compared with the example response curves shown in the manual. 

ULTRABRA
What does it mean when there is still a dip in the corrected curve - is that frequency still going to be a problem?  Why doesn't it flatten it out there too?

One important thing to be aware of is this.  The line shows level adjustments.  As I understand it, ARC also makes phase adjustments that, at low frequencies, can have a significant effect.  There is no visible feedback on this.  My displayed adjustments look contrary to the level adjustments that happen as I switch between enabled and disabled state.  I'm pretty sure this is because of the not-shown phase adjustments.