When the AD schema you are querying has been extended by an Exchange installation, likely you will obtain a DECODE error, and you will see in STDERR output this line:
20120419 105420.800 tb40 PDU 8936:50 ### ERR detect EPDU_TOO_LONG 126cc7 LDAP PDU length exceeds administrative limit 1048576 (DSMAX_LDAP_PDU_LEN)
In addition to other two lines reporting EIODECODEFAIL error.
Default value for the max pdu length is 1024 KB (1MB), but typically the schema once extended by Exchange is about 1.4 MB.
Increase the max pdu length to value 2048. From GUI in Global Parameteres -> Global Properties -> Max PDU Length
Note that it is also possible that higher values may be needed such as 4096.