From my own personal experiance it depends on intent. We've had an open mat policy forever and Shoji Nishio Shihan always encouraged his students to check out other arts...
How else does one learn what works and what does'nt??? How else does one make connections in the Spirit of Aikido???
Heck Stan Pranin had the good spirit to invite allot of qualified teachers from all corners on the Martial Arts Universe to the Aiki-Expo's and 99.9% of the time you saw teachers exchanging ideas and learning from each other. What the heck is wrong with that?
I have experianced many technical disagreements over the years here on Aiki-Web and other sites...and as a result met some pretty darn good folks when they dropped in or I went to visit them...Not once have I experianced any intent to harm or embarass me from anyone who showed up. Not once have I ever gone to someone's Dojo with ill intentions...
Stepping on someone else's mat should always be done in this spirit...
William Hazen