Yeah, I've got the same problem since I installed Mountain Lion and it's getting really ridiculous. In the last fifteen minutes my two year old MBP went to sleep at least five times. Could be more. Occasionally my notebook works fine for 30 minutes and then, bam, sudden sleep mode. Thus I can't really make out a pattern.
Here are a few more details: When my notebook goes to sleep it immediately wakes up again. So it doesn't stay in sleep mode but immediately wakes up due to, what Console calls, "Wake reason: EC LID0".
What all sleep modes have in common is the way my laptop goes to sleep (as seen in Console):
8/20/12 11:54:26.027 AM com.apple.time[11]: Next maintenance wake [Backup Interval]: <date: 0x7fed0ae3d910> Mon Aug 20 12:40:25 2012 CEST (approx)
8/20/12 11:54:26.027 AM com.apple.time[11]: Requesting maintenance wake [Backup Interval]: <date: 0x7fed0ae3d910> Mon Aug 20 12:40:25 2012 CEST (approx)
8/20/12 11:54:27.247 AM WindowServer[90]: Created shield window 0x3b7 for display 0x042731c0
8/20/12 11:54:27.000 AM kernel[0]: AirPort_Brcm43xx::powerChange: System Sleep
8/20/12 11:54:27.000 AM kernel[0]: hibernate image path: /var/vm/sleepimage
8/20/12 11:54:27.000 AM kernel[0]: sizeof(IOHibernateImageHeader) == 512
8/20/12 11:54:27.000 AM kernel[0]: kern_open_file_for_direct_io(0) took 0 ms
8/20/12 11:54:27.000 AM kernel[0]: Opened file /var/vm/sleepimage, size 4294967296, partition base 0x0, maxio 400000 ssd 0
8/20/12 11:54:27.000 AM kernel[0]: hibernate image major 1, minor 0, blocksize 512, pollers 4
8/20/12 11:54:27.000 AM kernel[0]: hibernate_alloc_pages flags 00000000, gobbling 0 pages
8/20/12 11:54:27.000 AM kernel[0]: hibernate_setup(0) took 0 ms
8/20/12 11:54:27.263 AM WindowServer[90]: device_generate_desktop_screenshot: authw 0x0(0), shield 0x0(0)
8/20/12 11:54:27.272 AM WindowServer[90]: device_generate_lock_screen_screenshot: authw 0x0(0), shield 0x0(0)
8/20/12 11:54:27.000 AM kernel[0]: en1: BSSID changed to 00:19:cb:9f:e1:b4
8/20/12 11:54:27.000 AM kernel[0]: wlEvent: en1 en1 Link DOWN virtIf = 0
8/20/12 11:54:27.000 AM kernel[0]: AirPort: Link Down on en1. Reason 8 (Disassociated because station leaving).
8/20/12 11:54:27.000 AM kernel[0]: en1::IO80211Interface::postMessage bssid changed
[VPN commands]
8/20/12 11:54:28.462 AM configd[18]: network changed: v4(en1-:192.168.1.34) DNS- Proxy- SMB
8/20/12 11:54:28.470 AM mDNSResponder[38]: DeregisterInterface: Frequent transitions for interface en1 (192.168.1.34)
8/20/12 11:54:29.000 AM kernel[0]: hibernate_page_list_setall start 0xffffff806712e000, 0xffffff80673b9000
8/20/12 11:54:29.000 AM kernel[0]: hibernate_page_list_setall time: 323 ms
8/20/12 11:54:29.000 AM kernel[0]: pages 970725, wire 126081, act 210879, inact 53, cleaned 0 spec 1, zf 151932, throt 0, could discard act 217420 inact 221354 purgeable 18355 spec 63 cleaned 24587
8/20/12 11:54:29.000 AM kernel[0]: hibernate_page_list_setall found pageCount 488946
8/20/12 11:54:29.000 AM kernel[0]: IOHibernatePollerOpen, ml_get_interrupts_enabled 0
8/20/12 11:54:29.000 AM kernel[0]: IOHibernatePollerOpen(0)
8/20/12 11:54:29.000 AM kernel[0]: encryptStart 132a0
8/20/12 11:54:29.000 AM kernel[0]: IOPolledInterface::checkForWork[4] 0xe00002eb
8/20/12 11:54:29.000 AM kernel[0]: PMStats: Hibernate write took 3 ms
8/20/12 11:54:29.000 AM kernel[0]: all time: 3 ms, comp bytes: 0 time: 0 ms 0 Mb/s, crypt bytes: 184160 time: 1 ms 147 Mb/s,
8/20/12 11:54:29.000 AM kernel[0]: image 0, uncompressed 0 (0), compressed 0 (0%), sum1 17c882bb, sum2 0
8/20/12 11:54:29.000 AM kernel[0]: wired_pages_encrypted 0, wired_pages_clear 0, dirty_pages_encrypted 0
8/20/12 11:54:29.000 AM kernel[0]: hibernate_write_image done(e00002eb)
8/20/12 11:54:29.000 AM kernel[0]: sleep
8/20/12 11:54:31.376 AM WindowServer[90]: handle_will_sleep_auth_and_shield_windows: no action for lock state 1
8/20/12 11:54:31.000 AM kernel[0]: Wake reason: EC LID0
8/20/12 11:54:31.000 AM kernel[0]: HID tickle 51 ms
8/20/12 11:54:31.000 AM kernel[0]: AirPort_Brcm43xx::powerChange: System Wake - Full Wake/ Dark Wake / Maintenance wake
8/20/12 11:54:31.000 AM kernel[0]: Previous Sleep Cause: 5
I already resetted the SMC, tried the caffeinate command, adjusted all the relevant settings and also tried the app Coffeine. Nothing worked, unfortunately. I also can't reinstall Mountain Lion right now as I don't have a good enough Internet connection and my notebook goes to sleep randomly. So yeah. And ideas?