Can’t recommend Get the Image plugin (by Justin Tadlock) enough. It’s among the best out there, very well maintained and neat code — more importantly, it does what you need, while giving you a whole slew of options.
How does it pull images?
Looks for an image by custom field (one of your choosing).
If no image is added by custom field, check for an image using the_post_thumbnail() (WP 2.9’s new image feature).
If no image is found, it grabs an image attached to your post.
If no image is attached, it can extract an image from your post content (off by default).
If no image is found at this point, it will default to an image you set (not set by default).