LimTS,
Yes, it can be done. You didn't specify whether you want to bridge IP or a non-routable protocol like, Netbios, LAT etc. If you want to bridge a non-routable protocol and route IP then you can configure transparent bridging on the router and that's all you need.
Instead if you want to bridge and route IP then you need to enable a feature called Integrated Routing and Bridging (IRB). This would enable the router to bridge and route the same protocol like IP.
I wouldn't say a big downside but anytime you bridge a protocol across the WAN, then you are sending broadcast traffic across the WAN which isn't ideal but then if you had a requirement that demands this solution then you would have no choice but to implement it.
This link should guide through the process of setting it up.
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca767.html#wp1002244
HTH,
Sundar
