When we started with v8 of NetLedger, there was no good way to limit the items on the customer portal to a specific customer.  The ideal way would be to have a field in under the Finance Subtab of a Customer to select specific item category or categories that would be available when you give access.

The way we acheived this in the past was to use price levels in a special way. You could limit customers to only specific price levels and if you created specific price levels instead of categories for items, you could assign unique price levels to the customer. As long as there was no base price used for the item, the items listed would only be those for the specific level assigned to the specific customer.