I'm transferring a new client over and unfortunately have had to go through some hoops with NSI regarding their domain name.

I was able to change the nameservers from their departing host to my own, but when I check whois I see that the IP address NSI has associated with my nameservers is incorrect. It appears to be an old set of IPs that used to belong to my nameservers.

So my question is: Is this going to cause a problem once the propagation is complete? Every registrar that I've dealt with in the past has always allowed you to plug-in the FQDN as well as the corresponding IP addresses (or the system has looked them up), but NSI only allows the FQDN and has somehow associated old IP addresses with them.

Anyone else run into this? Will it be an issue?

Here's a suggestion: log inside your account and use their
Manage Host Servers (I think that's what they call it) link to see
what IP is listed with your nameservers.

Next, email NSI and ask them to check their inside systems to see
what IPs are associated with your nameservers. If the part above
shows the correct IPs, chances are that's what they'll also see.

Additionally, check the Registry of the nameservers' extension and
search under "nameserver" to see what IPs are associated with
them.

Remember that the Registry shows authoritative records of both
domain names and nameservers, so ideally what shows up at the
Registry is what the registrar should show in theirs as well.

NSI's WHOIS is actually "cosmetic", meaning they don't really take
their WHOIS details seriously except for the registrant name.

Although the IP addresses of the nameservers in their WHOIS
show differently, it mostly won't matter as long as they have the
correct IPs in their internal database.