Question: We are currently using transactional replication in SQL Server 2008 to keep a secondary reporting server synchronized with a primary database server. This has been working weel and keeps some of the load off the primary server. Would it be possible to scale this solution to multiple reporting servers? We're expecting an increased load of read-only queries and it would be nice to be able to add reporting servers as needed.

The general idea was the following:

1. Each reporting server would use a "pull" subscription to get the data from the primary database publication. These reporting databases could be a couple of minutes behind the primary server without it being an issue.
2. The reporting servers would be NLB'd together. All read-only queries would be directed to the NLB which should spread the load across the servers.

Answer: 
Sounds like it would work, to me. As long as you're only doing queries to the NLB name. However, I would take a long deep look into the MS SQL deployment and architecture documentation to see if there's anything that says "build it this way, taking care that you do X and don't do Y" or "It won't work at all because of Frob."
Wouldn't it be a lot better to have multiple servers running Reporting Services, all of them querying a single Database Engine server?

There's no need at all for RS to use a DE on the same machine.

About NLBing Reporting Services: yes, you can, but it has some caveats. See <http://technet.microsoft.com/en-us/library/cc281307.aspx>.