The avr is capable of much higher data rates. I have gotten 1m baud out of the arduino uno with out any issues although I did not use the Serial library just port manipulation. The data makes it to my computer just fine I have wrote some programs that use serial at 1m baud and it works very well. I think that by not having the higher speeds available it may give beginners the impression that the avr is less capable than it really is. Since the arduino is clocked at 16mhz 1m baud has 0% error where as some of the more common baud rates such as 115200 baud have some slight error in speed so not only is high speed to be gained but also a higher chance of data integrity. I see no reason for this artificial limitation. Also speaking of the serial monitor if you have that open and unplug the arduino CPU usage goes up to 100% until you exit the serial monitor.
This is a well known issue, and as the IDE is open source too feel free to implement (nobody did it yet).
I do not know if the java libs that are underneath the IDE support higher baud rates.
Besides higher baud rates I would love to have any baud rate (there was this post of someone with a 7200 baud device)
You can always use a PC terminal program as your serial monitor, there are a ton of free ones available with many more features then the bare bone arduino serial monitor. I use Brey terminal, and you can keep it opened on your desktop as it has a connect/disconnect button so that you can easily free up the serial port for uploading with the IDE and then connect for monitoring.
Lefty
Not sure if /dev/tty can do oddball rates, I use screen. I don't even bother with the IDE anymore, I only use the tools. 