Hi,
It depends on the actual platform.
The following platforms support hot-swapping for similar network modules (i.e like for like replacement):
Cisco 3660
Cisco 3745
Cisco 3845
The following routers do not hot-swapping of network modules i.e. the routers must be powered down before installing network modules.
Cisco 2600 series
Cisco 2811, Cisco 2821, Cisco 2851
Cisco 3620
Cisco 3640
Cisco 3825
Hope that helps,
Paresh.