Does anybody know why an IXP needs to have an ASN? Will that ASN appear in the AS-PATHs or will it be transparent?
In its most basic form, an IXP is nothing more than a large switch, allowing many networks to exchange traffic without having to interconnect with every other network on the IXP, thus reducing costs of cabling and route rports. As it only provides layer 1 and 2 connectivity, the IXP does not require to have an ASN.
However, IXPs often do a bit more: they host a website, customer portal, mail and possibly other services. Since they often do not wish to depend on hosting by one of the connected networks, they can get an ASN to host those services in.
Many IXPs also provide some routing services. Some IXPs offer a route collector, which is basically a BGP speaker which only receives routes but does not advertise any, to collect statistics about the IXPs. To setup BGP sessions with all IXP members a ASN is needed, and private ASNs are not really useful due to the chance of duplicates.
Also, many IXPs offer a route reflector service. This service allows IXP members to setup a BGP session with a router managed by the IXP to exchange routes with other other connected IXP members. Of course, this route reflector requires a ASN as well. The route reflector is only used to exchange routes (and next-hops on the peering platform), it's not in the forwarding path (else you would need routers and conncetions capable of handling a large part of the entire IXP platform). As the route reflector advertises routes with next-hops set to other peers, you will not see it in the forwarding path.