Managing expectations correctly can be a full time job. Seth Godin recently posted on his blog that

most of the time, you’re challenged with this: high expectations that must be beat.

Over the years, I’ve worked with people that habitually over-promised and under-delivered. It’s a tough place to be and usually only ends in disappointment, mistrust and a short business relationship. I’ve often associated it with the dreadful reputation that sales people get, one I’ve always challenged whenever I hear it.

Be honest. There’s no way back from dishonesty, and certainly no way forward for a client to recommend you to others.