The documentation for r.mapcalc and NULL values is at:
http://hgeo02.geog.uni-hannover.de/grass/gdp/html_grass5/programming/raster/fpnv.html

In short, you probably need to use the isnull() function.  Otherwise, 
sometimes it is appropriate to use r.null to set the nulls to zero.
NULL doesn't behave as zeros did because NULL is suppose to represent 
"undefined" and operations using undefined will (by definition)
result in undefined.

I copied it here for you:


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Division by zero should result in NULL. 

     This is a non-backward-compatible change and should be 
         noted in the manual entry for r.mapcalc
	 as well as in any release notes. 

     Modulus by zero should result in NULL. 

     This is a non-backward-compatible change and should be 
	 noted in the manual entry for r.mapcalc
	 as well as in any release notes. 

     NULL-values in any arithmetic operation should result in NULL. 
     NULL-values in logical operations should be handled as follows: 
              NULL || true = true 
              NULL || false = NULL 
              NULL || NULL = NULL 
              NULL && true = NULL 
              NULL && false = false 
              NULL && NULL = NULL 
     NULL-values in function arguments should result in NULL. 

     Even though this is also true for the if() function, 
	 if() is spelled out here for clarification: 

     if(x) 
         NULL if x is NULL; 0 if x is zero; 1 otherwise 
     if(x,a) 
         NULL if x is NULL; a if x is non-zero; 0 otherwise 
     if(x,a,b) 
         NULL if x is NULL; a if x is non-zero; b otherwise 
     if(x,n,z,p) 
         NULL if x is NULL; n if x is negative; 
	 z if x is zero; p if x is positive 

     Exceptions: The (new) function isnull(x) returns: 1 if x is NULL; 
                 0 otherwise. The (new) function null() 
                 (which has no arguments) returns an integer NULL. 

     Non-NULL, but invalid, arguments to functions should result in NULL. 

     Examples: 
     log(-2) 
     pow(a,b) where a is negative and b is not an integer 

     Exception: sqrt(-n) returns -sqrt(n) 

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