Did some testing:
using whatsmyuseragent.com:
Browser: (Unchecked): Mozilla/5.0 (Linux; U; Android 4.1.1; en-us; Transformer TF101 Build/JRO03C) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Safari/534.30
(Checked): Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/534.24 (KHTML, like Gecko) Chrome/11.0.696.34 Safari/534.24
Chrome: (Unchecked): Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A293 Safari/6531.22.7
(Checked): Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A293 Safari/6531.22.7
Dolphin Browser HD: Android (Default): Mozilla/5.0 (Linux; U; Android 4.1.1; en-us; Transformer TF101 Build/JRO03C) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Safari/534.30
Desktop: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_3; en-us) AppleWebKit/533.16 (KHTML, like Gecko) Version/5.0 Safari/533.16
iPad: Mozilla/5.0 (iPad; U; CPU OS 3_2 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Version/4.0.4 Mobile/7B367 Safari/531.21.10
iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A293 Safari/6531.22.7
iPad: Mozilla/5.0 (iPad; CPU OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3
Firefox Beta (Using Phony Add On): Desktop Firefox: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20100101 Firefox/13.0
Android (Phone): Mozilla/5.0 (Linux; U; Android 4.0.2; en-us; Galaxy Nexus Build/ICL53F) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30
Android (Tablet): Mozilla/5.0 (Linux; U; Android 4.0.4; en-us; Xoom Build/IMM76) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Safari/534.30
iPhone: Mozilla/5.0 (iPhone; CPU iPhone OS 5_0_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A405 Safari/7534.48.3
iPad: Mozilla/5.0 (iPad; CPU OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3
Microsoft IE: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)
I went to Facebook.com with every one of those User Agent settings.
The following redirected to m.facebook.com:
Chrome [Unchecked]
Dolphin Browser HD [iPhone]
Firefox [Android (Phone)]
Firefox [iPhone]
The following defaulted to the desktop version of Facebook.com
Browser [unchecked]
Browser [checked]
Chrome [checked]
Dolphin Browser HD [Android(default)]
Dolphin Browser HD [Desktop]
Dolphin Browser HD [iPad]
Firefox Beta [Desktop Firefox]
Firefox Beta [Android (Tablet)]
Firefox Beta [iPad]
Firefox Beta [Microsoft IE]
I also went to NexusTablets.net with every one of those User Agent settings.
The following redirected to the mobile version of NexusTablets.net:
Chrome [Unchecked]
Dolphin Browser HD [iPhone]
Firefox [Android (Phone)]
Firefox [iPhone]
The following defaulted to the desktop version of NexusTablets.net
Browser [unchecked]
Browser [checked]
Chrome [checked]
Dolphin Browser HD [Android(default)]
Dolphin Browser HD [Desktop]
Dolphin Browser HD [iPad]
Firefox Beta [Desktop Firefox]
Firefox Beta [Android (Tablet)]
Firefox Beta [iPad]
Firefox Beta [Microsoft IE]
Conclusion: Some sites (like Facebook) will ONLY show the mobile site by default if the user agent shows that the device is a PHONE. The Stock Browser does not show itself as an Android Phone, but an Android Tablet. Some sites treat TABLET user agents to show the non-mobile version of the site.