It is now looking very likely that A Dance with Dragons is going to be split for its (eventual) mass-market paperback publication in the UK.
A previous commentator suggested that he'd heard from HarperCollins Voyager (who publish A Song of Ice and Fire in the UK) reps that this was the case, and I have been able to verify that this is the current likelihood.
A Dance with Dragons currently stands at 1,500 manuscript pages in length (well north of 400,000 words), just short of the 1,530 pages of A Storm of Swords. As previously reported, George R.R. Martin has five chapters left to complete and finalise, and these pages will take the book over ASoS's size. The good news is that, at the moment anyway, the American publishers seem able to publish the book in both hardcover and paperback in one volume with no problem at this size. Voyager are also planning to publish the book in one volume in hardcover and tradeback, but, like A Storm of Swords before it, it will have to be split for mass-market paperback.
This news is likely to be of interest to those UK readers planning on holding off on purchasing the book until the paperback comes out for cost reasons, as two paperbacks (at £7.99 each) will be more expensive than a tradeback (£15) or possibly even the hardcover (£20, but likely heavily discounted on release).
Apparently (not from official sources) the printers that HarperCollins utilise refuse to publish such a large book in one volume and contractual obligations prevent them from going to a different printer (such as Bantam UK's who put Steven Erikson's 1,300-page paperbacks out with no problem, or Pan Macmillan's, who have no problem publishing Peter F. Hamilton's gigantic tomes). Whilst precedented by the split of A Storm of Swords in the UK, this news is likely to frustrate UK fans of the series further.
To reiterate, the American publication of the novel appears to be holding at one volume, and the British initial release will also be in one volume. This applies to the British mass-market paperback release only.
Forgot to mention this, but HarperCollins Voyager are also publishing a new edition of A Game of Thrones in the Spring to tie in with the TV series (as Bantam already are). The new edition is more than just a rejacketing as it will feature a whole new typeset. Those with the existing UK copies of Thrones will know that the book has a fairly thick, ink-heavy typeset when compared to the later books in the series. An extremely minor change, but one that is quite interesting anyway. The new edition will also be in the larger 'B'-class paperback.
Voyager are also doing a special release of the entire series in e-book format, hopefully before ADWD and the TV series appear.