There are a lot of factors in play (Xbox 360 price drop, GT5 Prologue) but it's still interesting that the Xbox 360 has been outselling the PS3 in the UK since GTAIV was released.
http://www.eurogamer.net/article.php?article_id=143670
"Sales of Xbox 360 consoles are 18 per cent ahead of the PlayStation 3 in the UK since the launch of Grand Theft Auto IV, Chart-Track told Eurogamer today."
...
"The effect on driving hardware sales has also been more dramatic on 360 than PS3, according to Chart-Track’s figures. Comparing sales of both consoles for the four weeks prior to GTA IV’s launch to the four weeks since its launch, hardware units have risen on 360 by 47 per cent, compared to just eight per cent on PS3 - though it’s fair to point out that the release of GT 5 Prologue had already given PS3 a significant boost in the weeks leading up to GTA IV’s debut."
