Android 3.0 Honeycomb, will not connect to Mac OS X unless you install a helper program called "Android File Transfer" on your PC. "Android File Transfer is an application for Macintosh computers (running Mac OS X 10.5 or later) you can use to view and transfer files between your Mac and an Android device (running Android 3.0 or later)."
AFT provides third-party Media Transfer Protcol (MTP) support, which Mac OS X doesn't natively provide. MTP started off as a Windows Media Player technology, which's probably why Apple doesn't support it, but today it's represented by a standard USB device class and an extension of its precursor, PTP.
MTP support is definitely a step up as far as Android is concerned. Rather than 'turn on mass storage mode,' which locks local apps out of your SD card, MTP allows both the phone and the PC to access your phone's filesystem at the same time. Hopefully Medial Transfer Protocol will find its way to the smartphone version of Android in due course
Download: Android File Transfer (.dmg)