I predict a mere closing of weekly sales gap between the 360 and the PS3 in the UK, after the initial two-three week sales boost. I think the PS3 will be able to stay above the 360 by about 20-30k in the US for the rest of the year, not sure how long into 2010 that will last.
I think the Slim will have the best effects in mainland Europe and Japan, simply because these markets are already more open to the PS3 than the UK/US. It'll be interesting to see whether this will be able to pull back the weekly lead that the PS3 had over the Wii in Spain and Italy. I think this, along with the likes of FFXIII and GT5 will be able to close the gap between the PS3 and Wii in Japan for quite some time, with the PS3 outselling the Wii in Jap for the end of this year and the start of next (depending on when the two titles get released).
The success of the slim will also be heavily dependent on the quality and quantity of marketing it receives. So far, this generation, Sony have failed to deliver on at least one of these aspects with each campaign, let's hope that they get it right this time.
On another note, I do think that, given the PS3 slim and the price drops/adjustments on the 360, the Wii will never be able to attain and maintain that 50% marketshare limit that Nintendo fans have been hoping for oh-so-long. In fact, I think that the Wii's market share will drop to about 45%, perhaps lower, over the next 6 months*
*That being if Nintendo doesn't respond with other sale-drivers like price changes, extra bundling, and different SKUs.