Does being in a commited married life make it impossible to attain Nirvana? If not, how can one progress towards Nirvana? Married life is a form of attachment. If one is in a commited relationship he/she is bound to have a multitude of attachments like a wife, children, personal belongings, wealth etc. But Buddha has once said : 


  Costumes Do Not Mar Virtue:
  
  "Even though adorned, if living in peace
  calm, tamed, established in the holy life,
  for beings all laying force aside:
  one pure, one peaceful, a bhikkhu is he."
  
  Verse 142, DHAMMAPADA.


Need some verification as to how someone can be in a commited relationship and at the same time achieve nibbana.
how someone can be in a commited relationship and at the same time achieve nibbana

I had a dream recently in which my wife went out for the evening without me: I was jealous, craving her and self-centred (wanting her to pay attention to me instead), clinging (wanting to not to lose her, wanting to not lose or to repeat the happy experience/relationship I had earlier with her), and averse to the situation (not wanting to be left alone), and angry (with her for making the decision to go out without me).

Because I was jealous and so on I was unhappy. Even when she came home she still seemed unhappy and avoided me.

Then I thought (after I woke up), maybe it's because I was unhappy like that she wanted to go out without me. Who wants to spend time with some unhappy man? Can you even ever satisfy someone who is craving and clinging?

Our relationship would be better if I were happier in myself, more "unconditionally" happy ... or more possessed of equanimity and metta and less of ego and so on. Then when she came home I could say, "Oh, nice to see you, how are you?" and so on and be a better friend.

a commited relationship is bound to have a multitude of attachments

Perhaps it's helpful to distinguish between "attachment" and "responsibility".

For example maybe you have a responsibility to try to help your children, feed and educate them.

When I was first born my father wanted me to grow up to be a good cricket player: that was his ambition, he put a cricket bat in my cradle, etc. I tried cricket a few times (he played) but not very often. It's probably best that he wasn't too "attached" to the idea of me becoming a cricket player.

In this answer yuttadhammo wrote,


  In brief, duty and responsibility should be what dictate our worldly affairs, not concern or care.


advice for marriage partners

In the book I mentioned in this answer the author tries to summarize the Buddha's advice for laypeople. Chapter 6 of that book is about "establishing a marital partnership" and includes the following advice:

For him:


Show respect to her
Refrain from words that might hurt her
Practice faithfulness
Give up dominance
Respect her fondness for beauty


For her:


Work in an organized way
Handle family relationships
Be faithful
Take steps to protect the family wealth
Show skill and energy
Play an active role as his companion and guide


Te book mentions that a lot of Buddhist literature is written by and for monks, and that the role of lay people in society isn't the same as monks'.

Chapter 5 of that book suggests that your partner should be someone who might help your own spiritual progress and life.