Using other provider's ASN (was Re: Sprint and peering points)

On Sat, 31 March 2001, Jeff Loughridge wrote:

Sprint does not prepend its AS at public or private peering points. What other providers do with the AS Path at their ingress is not under our control.

Providers trying to "steer" traffic through their peers, upstreams, or exchange points is nothing new. However, this caught my attention. If Sprint isn't pre-pending its routes with Sprint's ASN, who is? It is generally considered ill-mannered to use someone else's ASN without their permission. Traditionally, pre-pending is done with the pre-pender's own ASN. If nothing else, to help track down the party when evil things happen.