Hi
I am running MPLS on some 2621XM's in our lab at work. The IOS i am running is
(C2600-SPSERVICESK9-M), Version 12.3(4)T4
the routers have 128/32 so your's will definitely run it. Not all MPLS functionality is there but they do support MPLS/MPLS VPN/MPLS TE. If you want things lile MPLS FRR (Fast reroute) you need a different router.
The Cisco site does not specifically say the 2621XM is supported so i wouldn't run this in a production environment but it should be okay in a lab.
As i say i am using 2621XM's so i can't definitely say 2611XM would work but i think it probably would.
HTH
Jon