Why would mysql "show global status" query be taking 15 minutes? I'm reviewing the slow log, and on one of my slaves the average time for SHOW GLOBAL STATUS is 914s.

Any idea how to determine the cause of this?
That sounds very unusual, but not surprising. All it takes to run SHOW GLOBAL STATUS; is the USAGE privilege, which is just general connectivity as stated in the last sentence of the first paragraph of the MySQL Documentation on SHOW STATUS:

This statement does not require any privilege. It requires only the ability to connect to the server.

Given these facts, there can only be two possible causes:
POSSIBLE CAUSE #1
In MySQL 5.1 and MySQL 5.5, values for global status are accessible via the INFORMATION-SCHEMA database

INFORMATION_SCHEMA.GLOBAL_VARIABLES in MySQL 5.5
INFORMATION_SCHEMA.GLOBAL_VARIABLES in MySQL 5.1

Here is the table layout:
mysql&gt; show create table information_schema.global_status\G
*************************** 1. row ***************************
       Table: GLOBAL_STATUS
Create Table: CREATE TEMPORARY TABLE `GLOBAL_STATUS` (
  `VARIABLE_NAME` varchar(64) NOT NULL DEFAULT '',
  `VARIABLE_VALUE` varchar(1024) DEFAULT NULL
) ENGINE=MEMORY DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

Notice it is a MEMORY table. Like MyISAM, all INSERTs, UPDATEs, and DELETEs into a MEMORY table require a full table lock (See Locking Granularity for MEMORY Storage Engine). There has to be some intermittent lock to populate values into this table. Since this table is local to the DB Connection, only the DB Connection is affected.
POSSIBLE CAUSE #2
User authentication may be blocked for a time due to host caching and DNS. Note paragraph 2 the MySQL Documentation on DNS Lookup Optimization and the Host Cache:

For each new client connection, the server uses the client IP address to check whether the client host name is in the host cache. If not, the server attempts to resolve the host name. First, it resolves the IP address to a host name and resolves that host name back to an IP address. Then it compares the result to the original IP address to ensure that they are the same. The server stores information about the result of this operation in the host cache.

If you do not have skip-host-cache and skip-name-resolve configured, please add these to my.cnf:
[mysqld]
skip-host-cache
skip-name-resolve

and restart mysql ASAP.
EPILOGUE
Cause #2 is a bit of a stretch since authentication should not trigger an entry into the slow query log.
Cause #1 is a lot more likely because if the global status is being loaded into a table, it is an SQL query with discernible passage of time.Thus, if the query's running time (in seconds) exceeds long_query_time, it will get recorded into the slow query log.