Hi,
I just made a quick and dirty test here using DT_Ints. The Main routine waits for PortB.0 going low, when it does it enables the TX interrupt. The TX interrupt service routine extracts characters from the buffer array and puts them in the TX register of the USART. When it sees a CR it disables the interrupt.
Code:
'****************************************************************
'*  Name    : HWTX.BAS                                          *
'*  Author  : Henrik Olsson                                     *
'*  Notice  : Copyright (c) 2010 Henrik Olsson 2010             *
'*          : All Rights NOT Reserved - use as you see fit.     *
'*  Date    : 2010-11-16                                        *
'*  Version : 0.0                                               *
'*  Notes   : Test for interrupt based USART transmit using     *
'*          : DT_ints-18 on a 18F4520                           *
'****************************************************************
DEFINE LOADER_USED 1                ' We're using a bootloader
DEFINE OSC 20                       ' 20Mhz x-tal is used
DEFINE HSER_BAUD 38400              ' Baudrate is 38400
CMCON = 7                           ' Comparators OFF 
ADCON1 = %00001111                  ' No analog inputs
TRISC.7 = 1                         ' RxPin is input
TRISC.6 = 0                         ' TxPin is output
TRISB.0 = 1                         ' PortB.0 is input
TxBuffer VAR BYTE[128]              ' Transmit buffer array.
TxPointer VAR Byte                  ' Pointer into the above array
INCLUDE "DT_INTS-18.bas"            ' Include Darrels interrupt engine.
INCLUDE "ReEnterPBP-18.bas"         ' And the stub needed for interrupts in PBP 
 
ASM                                 ; Set up the USART transmit interrupt.
INT_LIST  macro     ; IntSource,   Label,    Type, ResetFlag?
        INT_Handler    TX_INT,   _USART_TX,   PBP,    no
    endm
    INT_CREATE                      ; Creates the interrupt processor
ENDASM
Boot:
  HSEROUT ["Program start",13]        ' A dummy HSEROUT here to init the USART and show we're alive.
 
  ' Now load the array with something we want to send. The string is, in this case, terminated with a CR (13).
  ArrayWrite TxBuffer,["This is the TX Array Buffer and we're sending it with an interupt based routine",13]
 
Main: 
  If PortB.0 = 0 then                 ' PortB.0 going low is what
    TxPointer = 0                     ' Initialize pointer to beginning of array
@   INT_ENABLE   TX_INT               ; Enable USART TX interrupt  
    Pause 100                         ' Simple debounce for the button
  ENDIF
  Goto Main                           ' And do it again.
 
USART_TX:
  TXREG = TxBuffer[TxPointer]       ' Load character from array into USART TX register
  If TxBuffer[TxPointer] = 13 then  ' If that character was a CR we're done so we.....
@   INT_DISABLE  TX_INT             ; ...disable the any further USART TX interrupts.
  ELSE                              ' If the character was not a CR we.....
    TxPointer = TxPointer + 1       ' ...increase prepare to send the next character.
  ENDIF
@ INT_RETURN                        ; And exits the interrupt.
Seems to work here, try it out if you like.

/Henrik.