If you're an AT&T DSL or U-Verse and have AT&T mobile phone service, there's no adverse impact to you.
What if you're an AT&T DSL or U-Verse customer and using Verizon or Sprint's femtocell or T-Mobile's UMA tech (if T-Mobile is still even actively selling devices that take advantage of it)? In this case, more than likely, you won't be impacted either unless you regularly hit or get close to hitting your cap. ... so there's no downside, then, right? Wrong.
It's a slippery slope. If Verizon were to implement a cap on FiOS services and exempted its femtocell - paralleling the case at hand, the issue and arguments for and against such a policy would be the same as those present on this thread. What if Verizon went one step further and chose to not only exempt the traffic carried by Verizon Wireless' femtocells but also exempted traffic resulting from Redbox Instant (Redbox and Verizon Wireless' joint venture)? Netflix would be at a disadvantage and would likely be less popular in these areas (and thus, economically harmed) because customers would not risk overages from the use of Redbox Instant.
If the internet service provider wants to exempt a certain class of traffic, I think that's probably less cause for concern. To selectively pick entities in which that provider has a vested economic stake is self-dealing and most certainly indicative of anti-competitive behavior.
If AT&T eventually chooses to not asses a byte count on data used while on a femtocell, AT&T DSL or U-Verse users and users of AT&T' mobile have an immediate loophole to their data caps. Ensure their phone is using the AT&T femtocell and use it as an access point for more data intensive uses. I'm not sure how much the connection through AT&T's femtocell affects a user's mobile internet speed, but I imagine for users wanting to avoid overages, this would be of little inconvenience.