The N900 does not currently support Generic Network Access (GAN), which is often also referred to by its implementation, "Unlicensed Mobile Access (UMA)". This is a feature that, if implemented by a mobile provider, allows acess to a 3G/GSM network through a generic Internet (IP) connection. In the case of the N900, this would be an 802.11 Wi-fi connection. This allows voice calls and SMS in places that have poor or zero cellular signal reception (e.g. basements, shielded buildings). This also effectively bypasses the need to purchase a separate Femtocell.
GAN is currently supported by multiple operators, including T-Mobile and Orange. It is supported on multiple smartphones, including Nokia's 6301, 6086, 7510 (According to Wikipedia)
The protocol is defined in 3GPP TS 43.318 and 3GPP TS 44.318. I see no requirements that would prevent a software implementation of this feature in the N900.
Link to brainstorm discussion thread
(This has been discussed in a more general sense on this thread.)