I have no problem with a pastor speaking to a muslim organization. I would hope that he would use the opportunity to preach the gospel to them. I have no idea whether Warren preached the gospel or whether he issued "get-along verbage." As a preacher, I would LOVE to be able to address a group of muslims.... provided there are lots of body-guards present.
if warren states that muslims and christians worship the same deity, i would not conclude that when speaking to muslims he says something about repenting one’s sins to Jesus, committing one’s entire life to Him, and thereby finding personal salvation.
There is nothing wrong with addressing a Muslim group. However, there IS something VERY wrong with addressing a group associated with the original Jihadist terrorist organization, the Muslim Brotherhood, which gave birth to both HAMAS and Al Qaeda, after that group was implicated in the largest terrorism financing trial in US history.