From: United Kingdom
If you are using NLB on published servers, you need to configure your publishing rules to "forward the original client IP address" under the 'To:' tab. If not, all source IP addresses appear to be froma single source (ISA) and NLB affinity will send all requests to the same NLB node.
For this to work, you will need to make sure that the published servers are configured to return Internet traffic via the publishing ISA Server. This can be done via internal routing or by way of the default gateway definition on the published servers. YMMV depending on your networking setup...
Ideally, you remove NLB from the Exchange servers and upgrade to ISA Server 2006. Once upgraded you can then use a new feature called web server farm load balancing to achieve the same result, only better