Be aware that HDMI over standard copper cable is limited to 15 feet in lenght. Over that and you will have transmission problems. There are special active cables and fiber conversion cables than can transmit further.

My guess would be that the 'inwall' cable has a tougher outer protective layer in order to survive the stress of installation.

From WIKIPEDIA:
Cable length
The HDMI specification does not define a maximum cable length. As with all cables, signal attenuation becomes too high at a certain length. Instead, HDMI specifies a minimum performance standard. Any cable meeting that specification is compliant. Different construction quality and materials will enable cables of different lengths. In addition, higher performance requirements must be met to support video formats with higher resolutions and/or frame rates than the standard HDTV formats.

The signal attenuation and intersymbol interference caused by the cables can be compensated by using Adaptive Equalization.

HDMI 1.3 defined two categories of cables: Category 1 (standard or HDTV) and Category 2 (high-speed or greater than HDTV) to reduce the confusion about which cables support which video formats. Using 28 AWG, a cable of about 5 metres (~16 ft) can be manufactured easily and inexpensively to Category 1 specifications. Higher-quality construction (24 AWG, tighter construction tolerances, etc.) can reach lengths of 12 to 15 metres (~39 to 49 ft). In addition, active cables (fiber optic or dual Cat-5 cables instead of standard copper) can be used to extend HDMI to 100 metres or more. Some companies also offer amplifiers, equalizers and repeaters that can string several standard (non-active) HDMI cables together.

You'll have absolutely no problem running 1080p over long lengths of HDMI cable. I currently have a 50 foot run to my projector.

What you need to do is forget about the stores that want to charge $70 for a 6 foot HDMI cable just to bone the customer. Go to monoprice.com, their cables are excellent and still inexpensive. If you are a little worried about it, check out avsforum.com and it's 500,000 members who seem to always agree that monoprice is the way to go.

And no, I do not work for them, I am just happy to get HDMI cables for $8 instead of $70 like the guy at Circuit City and Best Buy wanted to sell me.

They sell in wall cables of larger gauge for longer runs, check it out.

Another vote for monoprice - they're reliable and cheap! HDMI cables of any length are a ripoff currently at big-box stores.

Granted, any cable at any length is usually a ripoff at retail stores, but right now, HDMI is especially so. They're banking on your "well gee it's new so it must be expensive, right?" instinct. Don't fall for it!

I haven't used Monoprice... I went with a 10M (33ft) HDMI cable from Blue Jeans and have no problems with it. AVS forum members seem to like both. The jacket on the Blue Jeans cable indicates a CL2 rating, so it would be fine in-wall.

The main difference is the rating of the outer jacket - in-wall cables have to have smoke-limiting properties that regular HDMI cabling doesn't have to have. The performance is largely the same. Paragraph 6 of the linked Audioholic article spells it out better.

Here is a link that might be useful: in wall cabling