The 3rd generation partnership project (3GPP) and 3GPP2 have standardized the IP multimedia subsystem (IMS) to provide ubiquitous and access network independent IP-based services for next generation networks via merging cellular networks and the Internet. The application layer Session Initiation Protocol (SIP), standardized by 3GPP and 3GPP2 for IMS, is responsible for IMS session establishment, management, and transformation. The IEEE 802.16 worldwide interoperability for microwave access (WiMax) promises to provide high data rate broadband wireless access services. In this paper, we analyze the SIPbased IMS registration signaling delay for 3rd generation (3G) cellular and WiMax networks with specific reference to their interworking architectures. We also explore the effects of different WiMax-3G interworking architectures on the IMS registration signaling delay.