Time zones can be an annoyance, even within the same country if you've got an agent who lives on the opposite coast. It can get worse with an agent in another country.
The big thing, however, is the difference between domestic vs. international sales. Your domestic sales will be in your agent's country of residence. If you're in the UK and your agent is in the US, then your domestic sales will be in the US. This means your agent will earn the 15% domestic commission on them, rather than the 20% international commission. Since the US market is actually larger, this can work in your favor.
Check into the differences in how taxes are handled, too.
Also be aware that a US "query" is different than the letter used in the UK. It's shorter, and doesn't usually include the standard 3 chapter sample. Instead, you've got about 350 words to hook the agent on your plot and characters and then you can maybe send them 5-10 pages in the email's body, if their agency allows it.
You really want to pop down to "Query Letter Hell" here on the forum and read through some queries that are being workshopped in there. It's in the "Share Your Work" area, which requires a password: "vista" for access. You can go ahead and read or comment on what's posted there, but you can't make your own thread until you've got 50 posts.
