I operate a small WISP and have 1 DSL and one DHCP link to the internet. Presently, I use a dual wan SOHO router and follow that with a Mikrotik router for control of my WISP network. I need to add another DSL to the mix. Does Mikrotik now have the capability to load balance traffic in the outbound direction so I can eliminate my "up front" router and just run all three lines into the MT?

yes

Is there an application note somewhere that describes how to do outbound load balancing on multiple WAN (DHCP and PPPoE) ports? What MT firmware version has this? I know it is not in 2.8.15 which I am running now.

Thanks.

You should be able to do it on 2.8 - not sure. Read up on policy routing - it's more than likely what you want.

2.9 has allot of additional features that might interest you as well irt redundancy and backups