There's no reason why they shouldn't be able to use it for Ethernet or Fast Ethernet connections. The in-line power delivered by Cisco's Patch Panel goes out over the non-Ethernet pairs (5,4 and 7,8) and does not interfere with any data traffic over the Ethernet pairs (1,2 and 3,6).
This is not standards-based inline power, though: only devices which are compatible with Cisco's inline power specification (Cisco IP Phones, Cisco Wireless Access Points) can use it.
If the customer does not need a full-size 48-port power panel, there's a company called Red Hawk (http://www.red-hawk.com) that makes inline power equipment for IEEE 48V, Cisco 48V, or proprietary 24V applications. I've used some of their equipment to centralize inline power distribution to a handful of Cisco IP Phones, where I could not cost-justify a Cisco inline power Ethernet switch or patch panel. (We still used a Cisco switch though.)
All the customer really misses out on by using NON-Cisco switches is the ability to have CDP tell the IP Phones what auxiliary VLAN they should be using. That, and being able to have CDP tell you which switch ports have Cisco IP Phones attached.
Hope this helps.
