Well, I got it working!

There's probably some magic command that needs to happen to put the 
drive into 1KB block mode, but I don't know what it is.  So I just 
created a new quirk entry that sets it to 512-byte block mode all the 
time.

Here's the changes, in case anyone else needs to get the same drive
running:

Index: /usr/src/sys/dev/scsipi/st.c
===================================================================
RCS file: /cvsroot/src/sys/dev/scsipi/st.c,v
retrieving revision 1.192
diff -u -p -r1.192 st.c
--- st.c        23 Aug 2006 17:19:32 -0000      1.192
+++ st.c        14 Oct 2006 22:17:44 -0000
@@ -316,6 +316,15 @@ static const struct st_quirk_inquiry_pat
                 {0, 0, 0},                             /* minor 8-11 */
                 {0, 0, 0}                              /* minor 12-15 */
         }}},
+/* PRG */
+       {{T_SEQUENTIAL, T_REMOV,
+        "Seagate STT3401A", "hp0atxa", ""},    {0, 0, {
+               {ST_Q_FORCE_BLKSIZE, 512, 0},           /* minor 0-3 */
+               {ST_Q_FORCE_BLKSIZE, 512, 0},           /* minor 4-7 */
+               {ST_Q_FORCE_BLKSIZE, 512, 0},           /* minor 8-11 */
+               {ST_Q_FORCE_BLKSIZE, 512, 0}            /* minor 12-15 
*/
+       }}},
+/* PRG */
  };



----------------------------------------------------------------------
|   Paul Goyette   | PGP DSS Key fingerprint: |  E-mail addresses:   |
| Network Engineer | FA29 0E3B 35AF E8AE 6651 |  paul@whooppee.com   |
|                  | 0786 F758 55DE 53BA 7731 | pgoyette@juniper.net |
----------------------------------------------------------------------