What effects does double-NAT have on VoIP? What happens to the traffic in this type of situation and what conditions can occur as a result of it?
As @Ricky Beam indicated, you should have no issues other than delay with fully-functional, SIP-aware NAT devices. This is known as ALG (Application Layer Gateway) on some lower-end network devices and SIP Fixup or SIP Inspection on different Cisco firewall platforms depending on software version.
Generally, the recommendation on not using ALG comes from poor implementation of either ALG itself or the client not putting its address into the SIP packets for it to be found and updated with the NAT'd address. On higher-end network devices, this generally works though you may run into a bug as I did in a recent question with an older Cisco PIX version.
Any type of ALG/SIP Inspection requires symetrically routing especially if implemented in a stateful firewall so the NAT'd addresses can get reversed on their way back to the client sitting inside the NAT device.