Serial.print etc one the earlier Arduino 022 will BLOCK. That is, the program will not continue until the last character is sent.
On the Arduino 1.0 all serial outputs put in a buffer and the interrupts will empty the output buffer as fast as possible. Thus a simple Seril.print or two will not block, but if you fill the buffer (64 characters) faster then it can empty, then the Serial.print will BLOCK for a while.
You can partially test if this is happening for you by increasing or decreasing the baud rate.