BPM's are typically viewed as a competitor to Mendix. Mendix microflows are based off of BPMN and allow for you to build pretty much anything you can get from a BPM tool. Do you have any examples of processes that need to run somewhere else? Mendix has plenty of long running process ability, the system just needs to be architected properly.
Mendix has a white paper on BPM and aPaaS co-existing and leveraging eachother:
http://ww2.mendix.com/understanding-your-needs-for-bpm-apaas.html
Also, Mendix is not even mentioned on Gartner's iBPMs MQ - so obvioulsy they see a difference too.
Just wanted to have my story straignt before opening my mouth in front of a prospect.
Thanks for your reply.