Same problem here. I can play MP between 5 - 20 minutes before it freezes and when I start the task manager a message pops up saying "Arma 3 has stopped working". Looking at the log files I found the same problem as described by Shiverlock (sample of the relevant portion below). This happens since the latest (stable) update, no problems with the previous ones. Error occurred with both the latest normal and beta drivers for my graphics card (ATI HD 5850).

The content of my appdata folder with the data from multiple crashes can be found at: https://www.dropbox.com/s/2i4ifj4hncsc9yq/Arma%203%20Alpha.zip

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Exception code: C0000005 ACCESS_VIOLATION at 6BA721CB
Allocator: D:\Program Files (x86)\Steam\steamapps\common\Arma 3\dll\tbb4malloc_bi.dll
graphics: D3D11
resolution: 1920x1080x32
Addons:
[...]
Mods: A3
Distribution: 0
Version 0.60.106070
Fault time: 2013/06/06 23:58:59
Fault address: 6BA721CB 01:001511CB D:\Program Files (x86)\Steam\steamapps\common\Arma 3\PhysX3_x86.dll
file: co40 AW Invade & Annex 2.86.1.15 REVIVE (__CUR_MP)
world: Stratis
Prev. code bytes: 85 C0 74 1D 8B 50 40 F6 42 4C 01 75 14 8B 40 60
Fault code bytes: F6 40 2C 01 75 0B 51 50 57 E8 97 FF FF FF 83 C4