A slew of articles about retail and digital signage in recent days:

has a well-noted article on the realtive strength of in-store and online media growth.
Quote:

The study finds shopper marketing has grown from 3% of the overall marketing budgets of the 19 package-goods manufacturers surveyed in 2004 to 6% this year. The manufacturers expect it to reach 8% of marketing budgets by 2010.

That puts the compound annual growth rate for their shopper-marketing spending at 21%, faster even than spending on internet advertising (rising 15% annually) and far faster than the 2% growth projected for spending on such traditional media as TV, print and radio.

reports that InStore Marketing Institute and Nielsen's PRISM consortium will syndicate its reporting on traffic at retail, in an attempt to "make stores a measurable marketing medium".
Some useful tidbits:

-- Traffic and Sales - P.R.I.S.M. shows that category transactions alone
are not a reliable indicator of category traffic, counter to
conventional wisdom. Two-thirds of shoppers who visit the salty snacks
section in a food store make a purchase. Traffic is far heavier past
the dairy case but the "closure rate," or number of shoppers who make a
purchase, is much lower for dairy products.

-- Demographics - Only 13 percent of food shopping trips are with kids,
but P.R.I.S.M. data shows shoppers put more in their baskets overall
when children are with them. This dynamic is constant throughout the
week and across most top product categories, including hair care and
water. Seasonal items are two and a half times more likely to be
purchased when kids are present. Interestingly, the presence of kids
on the shopping trip has little impact on candy sales.

-- Marketing Influences - Shoppers are communicated with frequently in
stores: the average number of individual pieces of marketing stimuli in
a grocery store is about 3,500 and larger store formats, such as mass
merchandisers, have over 5,000 stimuli. A typical drug store has
roughly 2,300 individual pieces of marketing stimuli.

Reveries.com posts result of a survey on brand-building at retail. Interesting results for In-Store Media, though the whole endeavor is still kind of an uphill battle.

Link

Wirespring's commentary here.

Quote:

[T]he overwhelming majority of respondents -- 83.2% -- feel that retail venues are a good place to build brand identity... However, only 12.4% indicated that retailers were generally helpful with the process. Far more -- 30.8% -- suggested that retailers tend to hinder their brand-building activities, or at best, move between periods of helping and hindering (41.4%). On the flip side, manufacturers fared little better. Only 28% felt that manufacturers were more likely to help than hinder. While far better than the opinion of retailers, this result is still pretty poor.

This one's about static screen networks like WalMart TV and Tesco, but I'd bet it applies to cart-based displays as well.

Quote:

Tesco's marketing partner Dunnhumby recommends five seconds for screens placed in the main shopping aisles, because the power of these "alerts" rests not in their creativity but in their proximity to purchase. The alerts reported to be most effective are those for price-off events and new or seasonal items.

Whereas a POP display or an endcap is designed to be shopped, walked past, seen from a distance, examined up close, and constantly navigated around, a piece of content on a digital sign often must be impactful, but fleeting. Take more than a few seconds to make your point, and lose the shopper's attention. Repeat yourself too often, and your message becomes annoying.