Anon - an interesting question and one that we'll be addressing at the October careers session "<a href=http://events.top-consultant.com/UK/careerconference.aspx?ID=421>Your career at a crossroads – which way next?</a>"
I would say your single biggest obstacle to returning will be that recruiters are understandably suspicious re. how serious contract consultants are about returning to perm work. There are 2 key reasons for this. Firstly, contractors broadly speaking earn double what they could earn in a similar permanent role (in part obviously to reflect the greater risk that they will have periods when they are paid nothing), which begs the question of "why would you return?".
But also lots of contractors approaching the end of their contract will BOTH look for new contract assignments AND start applying for permanent roles - with the latter a back-up plan in case no new contract assignment is secured. Since recruiters have therefore been on the receiving end of lots of applications from contractors that turned out not to be serious applications, they are understandably wary of all contractors. So your challenge will be to convince recruiters that you're not another time-waster but are serious about returning to a permanent consulting career.
On a more positive note, one possibility contracting may offer you is the chance to do some contract work for one of the consulting firms you'd actually like to join permanently. It can be a great way of bypassing the permanent recruitment process if you can get to work with the firm in a contract capacity, gain some real advocates within the firm and then propose becoming a permanent member of staff as a means of improving the profitability of the projects you are working on.
Of course you may well decide that you love the contractor lifestyle and never look back...
Hope this helps and good luck