<p>We're looking into developing a product that would use Amazon's cloud tools (EC2, SQS, etc), and I'm curious what tips/gotchas/pointers people that have used these technologies have.</p>

<p>One tip/whatever per post, please.</p>

An important concept to grasp: the file system your EC2 instance lives on while it's running
is not persistent.
There are tools/services available that let you mount file systems backed by S3 storage, or you can upload to S3 or other storage service from the instance, but when an instance closes the associated file system is no more.
As for tools, I've found Amazon's tools to be great, but you should probably be comfortable with the command line if you're taking this route.