In regards to using Cat5e for speaker wire, my installer still plans to use standard speaker wire for tying speakers together. He is not ready to commit to cat5e. He only saw it used for speakers in a demonstration. The downsides still have not been evaluated.
The whole structured wire issue, as stated by others, can get rather pricey. The biggest trouble is figuring out what is needed. Ideally each room should have two separate belden lines (2cat5 and 2RG-6) at different locations with homeruns to the main distribution box. Then the multiple lines for phone and RG-6 need amplifiers etc. What will the future hold? State of the art today, may be laughable in 10 years. Could a great wireless data transmiter method be developed? Wireless is available but probably not yet that good.
I plan to just use single cable runs to most rooms instead of 2 runs.
Also consider running conduit to different areas so that future wiring can be passed easily. This can be done with 2" pvc pipe but I am using a flexible 2" tubing called Carlon. Runs about 2$/foot. Run conduit to the attic, media areas, home theatre, family room etc. wherever you might need future wiring as technology and wiring needs change.