Especially during the northern hemisphere summer, students and postdocs commonly spend periods of weeks or months on exchange at foreign institutions. It's a fundamental part of how particle physics operates, with the science knowing few national boundaries and most modern experiments involving international collaboration. Students typically learn new skills that are transferable to their home experiments, gain a better understanding of how international collaboration works, and are able to share their specialized knowledge with other students and working scientists.
One example is the upcoming exchange program between the US Department of Energy and the National Institute for Nuclear Physics-Italy (INFN). In it, US students will spend 10 weeks at one of the INFN laboratories or sites, and a companion program will bring Italian students to DOE national laboratories.
If you are a US student interested in spending the summer in Italy (stipend provided), or know somebody who would, more details are at the DOE/INFN Summer Exchange Program for 2009 Web site.
There are many other similar programs that exist between countries, institutions, or laboratories, and if you know of any, feel free to add links to them in the comments below.
