This is the oldest trick in the "How to Start a New Business on the Cheap" book ... find someone that will provide what you need up front at no cost and offer them something in the future that may or may not happen.
Does that mean it is a guaranteed scam and they wouldn't send you clients? No ... but whether it's on purpose or simply that they don't end up receiving much business to pass along, the odds of you receiving the amount you would have received or more are slim.
As it has already been mentioned, if you are interested in exchanges like this, make sure there is some sort of equal arrangement. If they are a host as Linda said, I would think that offering you free hosting would be a simple way to pay you back for part of the design, or perhaps working out a half and half agreement where you would at least receive some income from the project.
I would also inquire with them on how they found you to start with. This could shed some light on whether they are serious or not. If they just randomly did a search for designers and contacted you with no referrals or anything to go on, offering you this "excellent" deal to send you all this business ... seems odd that a serious business would want to form an agreement with someone they don't even know anything about. Doesn't seem like a good business model to me. If they came from a referral, then perhaps I'd give it more thought. But a random company that found me and offered to send me all of their design business even though they've never worked with me (especially if they claim they will be marketing the web design side for me).
Also, I wouldn't ever consider forming a partnership / arrangement like this without working with the company first anyway. Who knows how well you would work together, what sort of communication you would have with them. They may be horrible at selling to the client or have too high expectations or late to send payment, etc. I would tell them that you would be willing to work with them on the next client they receive and see how the process goes. If it works well, then you can discuss an arrangement for their website. Most likely they will claim they can't do anything until their site is up (which any good business can still find clients without a new / fancy website), so there are even more red flags to show up.
Pretty much one of those "if it sounds to good to be true, it probably is". Another option I've had proposed to me is setting up an e-commerce site for free and I would receive a portion of the profits from every order placed. It always sounds like a GREAT way to make easy money ... but there really isn't "easy money" out there.
Of course that's just my philosophy on it. To each their own.