The shift to G20 only reflects the reality of the world. The 19 nation member plus the EU reportedly represent 90% of the world's GDP. It includes the new rising economies and according to the NYT:
The member countries are Argentina, Australia, Brazil, Britain, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey and the United States. The European Union is also a member, represented by the rotating council presidency and the European Central Bank.
It makes sense to have this group meet rather than the traditional big industrial countries of the G7, United States, Britain, France, Canada, Italy, Germany and Japan. We need to adjust to the reality that growth of economies throughout the world affects us all.
The second event reflects Obama's desire to approach problems with multiple allies rather than the show Bush put on to justify the desire to invade Iraq. Obama is trying to enlist a number of countries including Russia and China to put pressure on Iran to come clean about its nuclear capabilities and comply with international nuclear proliferation treaties. Hopefully the international community will respond and not be affected by its experience regarding Iraq.