The 5224 is almost strictly a Layer-2 switch - it has some limited CoS capabilities, but that's about it.

With the 5224, you would still need either a layer-3 device capable of supporting IEEE 802.1Q VLAN tagging, or you'll need to configure port overlapping, which would allow non-802.1Q compatible routers to forward traffic destined for the internet from any VLAN, for example, while preventing the internal VLANs from communicating with one another.

 

Do Dell offer any gigabit switches with Layer-3 functionality, or have plans to?

Dell has just released a Layer 3 switch that is an all gigabit solution.  Please refer to the following URLs for more details.

http://www.dell.com/Layer3

http://www.dell.com/networking