Hey that idea with the custom baseboard Tmsorosk suggested IS a really slick way to do it, provided you aren't using any big 'ol fat cable. It will be costly though. I had that done last April 2010 when the wife and I ripped out all our (ugly, dirty, smelly, 18 year old) carpet from "hell". We put in Oak hardwood floors throughout, and custom made 4 1/2" tall solid oak baseboards. We had the flooring contractor router a channel on the backside of the B.B. and then neatly tucked a 40 foot long run of Audioquest Diamond X3 interconnects (xlr connectors) into it. Once the B.B.'s were installed it looked super clean!
Problem was, it DID cost us a chunk 'o change to have that done. Like $2500 for the custom B.B., another couple hundred bucks for the "extra" router work, and of course the labor to install the whole shootin' match. So we ended up spending over 3 grand when it was all said and done, albiet NOT all just for the subwoofer cable. But pricey none the less. Probably would have been easier to try out the "wireless" option again? Aaronmadler has a great answer with the Outlaw Audio link. And it's very reasonably priced. Go check that out. If I had bothered to do my research prior to putting in new wood floors & custom B.B.'s, I might have gone that route, and saved a bunch of money! (money better spent on new music, good Scotch, import beer, and cigars!)