Note: I'm not set on the flag names.  I figured these would be
least likely to collide, but I haven't found -k or -u used anywhere
else with a trivial search of linux systems. I'm sure there are
some somewhere though.

Here's how it looks:
peter@daintree[ 2:23PM]~-1019> uname -K
1000055
peter@daintree[ 2:23PM]~-1020> uname -U
1000054

ie: I've got an out of sync userland and kernel on this machine.
Compile, library, includes etc is at 1000054 and the kernel is later.

Index: uname.1
===================================================================
--- uname.1	(revision 256232)
+++ uname.1	(working copy)
@@ -36,7 +36,7 @@
 .Nd display information about the system
 .Sh SYNOPSIS
 .Nm
-.Op Fl aimnoprsv
+.Op Fl aiKmnoprsUv
 .Sh DESCRIPTION
 The
 .Nm
@@ -55,6 +55,10 @@
 were specified.
 .It Fl i
 Write the kernel ident to standard output.
+.It Fl K
+Write the
+.Fx
+version of the kernel.
 .It Fl m
 Write the type of the current hardware platform to standard output.
 .It Fl n
@@ -70,6 +74,10 @@
 to standard output.
 .It Fl s
 Write the name of the operating system implementation to standard output.
+.It Fl U
+Write the
+.Fx
+version of the user environment.
 .It Fl v
 Write the version level of this release of the operating system
 to standard output.
@@ -91,6 +99,7 @@
 .Sh EXIT STATUS
 .Ex -std
 .Sh SEE ALSO
+.Xr getosreldate 3 ,
 .Xr sysctl 3 ,
 .Xr uname 3 ,
 .Xr sysctl 8
@@ -104,3 +113,10 @@
 The
 .Nm
 command appeared in PWB UNIX.
+.Pp
+The
+.Fl K
+and
+.Fl U
+extension flags appeared in
+.Fx 10.0 .
Index: uname.c
===================================================================
--- uname.c	(revision 256232)
+++ uname.c	(working copy)
@@ -54,6 +54,8 @@
 #include <stdlib.h>
 #include <unistd.h>
 
+#include <osreldate.h>
+
 #define	MFLAG	0x01
 #define	NFLAG	0x02
 #define	PFLAG	0x04
@@ -61,10 +63,12 @@
 #define	SFLAG	0x10
 #define	VFLAG	0x20
 #define	IFLAG	0x40
+#define	UFLAG	0x80
+#define	KFLAG	0x100
 
 typedef void (*get_t)(void);
 static get_t get_ident, get_platform, get_hostname, get_arch,
-    get_release, get_sysname, get_version;
+    get_release, get_sysname, get_kernvers, get_uservers, get_version;
 
 static void native_ident(void);
 static void native_platform(void);
@@ -73,11 +77,13 @@
 static void native_release(void);
 static void native_sysname(void);
 static void native_version(void);
+static void native_kernvers(void);
+static void native_uservers(void);
 static void print_uname(u_int);
 static void setup_get(void);
 static void usage(void);
 
-static char *ident, *platform, *hostname, *arch, *release, *sysname, *version;
+static char *ident, *platform, *hostname, *arch, *release, *sysname, *version, *kernvers, *uservers;
 static int space;
 
 int
@@ -89,7 +95,7 @@
 	setup_get();
 	flags = 0;
 
-	while ((ch = getopt(argc, argv, "aimnoprsv")) != -1)
+	while ((ch = getopt(argc, argv, "aiKmnoprsUv")) != -1)
 		switch(ch) {
 		case 'a':
 			flags |= (MFLAG | NFLAG | RFLAG | SFLAG | VFLAG);
@@ -116,6 +122,12 @@
 		case 'v':
 			flags |= VFLAG;
 			break;
+		case 'K':
+			flags |= KFLAG;
+			break;
+		case 'U':
+			flags |= UFLAG;
+			break;
 		case '?':
 		default:
 			usage();
@@ -153,6 +165,8 @@
 	CHECK_ENV("m", platform);
 	CHECK_ENV("p", arch);
 	CHECK_ENV("i", ident);
+	CHECK_ENV("K", kernvers);
+	CHECK_ENV("U", uservers);
 }
 
 #define	PRINT_FLAG(flags,flag,var)		\
@@ -176,6 +190,8 @@
 	PRINT_FLAG(flags, MFLAG, platform);
 	PRINT_FLAG(flags, PFLAG, arch);
 	PRINT_FLAG(flags, IFLAG, ident);
+	PRINT_FLAG(flags, KFLAG, kernvers);
+	PRINT_FLAG(flags, UFLAG, uservers);
 	printf("\n");
 }
 
@@ -243,9 +259,24 @@
 NATIVE_SYSCTLNAME_GET(ident, "kern.ident") {
 } NATIVE_SET;
 
+static void native_uservers(void)
+{
+	static char buf[1024];
+
+	snprintf(buf, sizeof(buf), "%d", __FreeBSD_version);
+	uservers = buf;
+}
+static void native_kernvers(void)
+{
+	static char buf[1024];
+
+	snprintf(buf, sizeof(buf), "%d", getosreldate());
+	kernvers = buf;
+}
+
 static void
 usage(void)
 {
-	fprintf(stderr, "usage: uname [-aimnoprsv]\n");
+	fprintf(stderr, "usage: uname [-aiKmnoprsUv]\n");
 	exit(1);
 }