When hyperterminal comes up and you hit enter, you should be seeing a login prompt, not an OK prompt. You are indeed sending a break signal - albeit involuntary - to the box and bringing the whole thing to a halt.

Sending a break signal on the system console causes the OS to drop into firmware mode and issue an "OK" prompt. Want the OS to start running again? Just type this command:
go

The reason is that Sun thought it would be cool if you could drop the machine to firmware mode by just unplugging and reconnecting the system console. It is possible for the OS to change this behaviour a second or two after a reboot, but there is still a very tiny window during boot where a break will work.

For solaris 2.6 get patch 105924-10 or later
For solaris 7 get patch 107589-02 or later
After solaris 7, no patch is needed.

Then modify /etc/default/kbd to have line like:
KEYBOARD_ABORT=alternate
Then after next reboot, you drop to firmware with ~^b (tilde, control-b)

There is also a kbd command to do this on the fly.

Ahhhhh.... Thanks a lot, I will mess around with that and see how it works.

You can plug-unplug terminals into any Sun box and it's not going to go to the ok prompt just for that.
The problem used to be that, if you powered off the terminal before disconnecting the serial cable, or powered on the terminal after connecting the serial cable, that would send a break signal to the console, thus bringing the box to the ok prompt.

Hmm, have you ever tried it when the console is a keyboard? Also the electrical glitch of unplugging a serial console can do this and as Perderabo already mentioned it was patched so the key sequence could be a little more complex resloving many of these issues. I have personally seen this happen in the past 12 months with the standard break enabled.

Literary hundreds of times using workstations, laptops, wyse terminals, xyplexes, etc. to connect to the serial console, for years.