Q:

When is the best time to collect login credentials during registration?

On a long, multi-screen registration flow, is it better to collect login credentials at the start, at the end, or when the user chooses to save their application and return to it later?

A:

I have never found multi-paged registration processes to be required.
Most of the time I opt for a 'progressive disclosure' model where the user is only asked for details when they are needed for a particular operation.
Lets say that you want to capture the users email, name, postal address, credit card details, and date of birth.
If you ask for it all up front there is a strong chance that they will refuse, get bored, get frustrated or find some other reason to abandon the process. 
If, however, you just ask them for an email and password combination and then show them a profile with all the spaces then they can choose what to add and when. If they decide to buy something then you ask them for their credit card details, name and postal address. You might have an offer that they can only qualify for on their birthday - They'll have to go and add their date of birth... Basically you only ask for the details when the are needed to complete the task in hand. 
The process can be speeded up by offering incentives for complete profiles (as stack exchange does) or by showing reminders that the users profile is incomplete (as Linked in does).
The point is that once the user has login credentials they can return when they like to add items to their profile or profile items can be collected during other processes and added to the profile.
In other words: Always set the login credentials first.

