UEFA's rules are slightly opaque to me, so I can't speak with anything approaching authority on the matter, but an article by ESPN Soccernet is claiming that the reason Chelsea have been so slow to complete the David Luiz transfer - and why they've been hesitant to spend ludicrous sums in general as of late - has been owner Roman Abramovich's fear of running afoul of the new Fincanial Fair Play laws, which state (probably) that teams must spend inside their means by the 2012/13 season or risk being disqualified from UEFA competition. In Chelsea's case, assuming that the current form is but a blip, that means no Champions League, one of the major pillars of funding for the club. That would be... bad.
Since transfer fees aren't really done as lump-sum payments very often, any buy now would have to be paid for over the next few years, and spending big this January may well have an effect on the club's bottom line two years from now. ESPN's source on the Luiz negotiations has this nugget for us today:
There is a deep reluctance on the part of Chelsea to pay the full amount demanded by Benfica. The reason is simple ... UEFA's new Financial Fair Play regulations come into force in a year or so, but clubs buying now and paying later will be hit hard if they find they are spending more than they are earning when those rules come in.
-Source: ESPN Soccernet.
Of course, it's not like owner Roman Abramovich couldn't just do things in lump sum payments anyway, so it's hard for me to see why on the face of things this should matter as long as the money doesn't get moved to some later date. In fact, shouldn't the impending rule change be encouraging clubs with vast cash reserves to spend as much money as fast as they can, knowing that this advantage will be denied to them in a few fairly short months? Obviously, managing the wages is a concern, but Chelsea are making strides there already - but it does seem bizarre that transfer fees would have a major impact on their strategy*.
My personal feeling on the Luiz deal is that it's taking so long because Chelsea are doing things properly rather than simply bowing to Benfica's opening demands. The Blues are still chasing the big name players - there's a strong link to Alexis Sanchez as well, and he'd hardly be cheap - but they're don't appear to be doing so out of sheer, lunging desperation. Due diligence seems to me a more likely explanation for Chelsea's slow movement this January than the club living in fear of Michael Platini's new justice.
Update: Here is a comment that explains the Fair Play rules and how they may or may not affect the club.
*Disclaimer: I am trained as a structural/biomechanical engineer, not as an accountant, so I have no idea how this black magic money business all actually works.