Advice regarding short term assignments in the USA please?

Hello all, I am an experienced professional engineer (Mechanical) in the UK. I am 50 years old and I work as a contract mechanical engineer taking short term assignments (usually between 4 and 12 months) in either design offices or chemical process type manufacturing facilities. I am presently working on a large Petrochemical Plant project here in the UK which I expect to complete around August 08. I would like advice regarding the possibilities (or lack of!) there are to work in the USA on short term assignments. I see on the web that there is a lot of work in the Houston TX area, for example, with the large design houses and operating companies alike. Do these companies take contract engineers on short term assignments and what is the usual route into these roles, employment agencies for example? I am experienced at working overseas but not in the USA. Thank you.