You can very easily find out the running threads in the database (e.g. like a MySQL SHOW PROCESSLIST) with db.currentOp.

> db.currentOp();
{ "inprog" : [ ] }

No much happening, however under some load you can see

> db.currentOp();
{
	"inprog" : [
		{
			"opid" : 27980,
			"active" : true,
			"lockType" : "write",
			"waitingForLock" : false,
			"secs_running" : 0,
			"op" : "insert",
			"ns" : "olympics.olympic_athlete",
			"client" : "127.0.0.1:63652",
			"desc" : "conn"
		}
	]
}
> db.currentOp();
{
	"inprog" : [
		{
			"opid" : 57465,
			"active" : true,
			"lockType" : "write",
			"waitingForLock" : false,
			"secs_running" : 0,
			"op" : "insert",
			"ns" : "olympics.olympic_athlete_affiliation",
			"client" : "127.0.0.1:63653",
			"desc" : "conn"
		}
	]
}

I was able to see these when I was Bulk Loading Data

The HTTPConsole at http://localhost:28017/ (for default installation) also shows you all client connections as well as more information per thread, database uptime, replication status and a DBTOP for recent namespaces. For example:

mongodb mactazosx.local:27017

db version v1.4.3, pdfile version 4.5
git hash: 47ffbdfd53f46edeb6ff54bbb734783db7abc8ca
sys info: Darwin broadway.local 9.8.0 Darwin Kernel Version 9.8.0: Wed Jul 15 16:55:01 PDT 2009; root:xnu-1228.15.4~1/RELEASE_I386 i386 BOOST_LIB_VERSION=1_40

dbwritelocked:  0 (initial)
uptime:    2851 seconds

assertions:

replInfo:

Clients:
Thread	OpId	Active	LockType	Waiting	SecsRunning	Op	NameSpace	Query	client	msg	progress
initandlisten	0		1			2004	test	{ name: /^local.temp./ }	0.0.0.0:0
snapshotthread	0		0			0			0.0.0.0:0
websvr	18		-1			2004	test._defaultCollection	{}	0.0.0.0:0
conn	83741		-1			2004	olympics.olympic_host_city	{}	127.0.0.1:63268
conn	83739		0			2004	?	{ getlasterror: 1.0 }	127.0.0.1:63756

time to get dblock: 0ms
# databases: 3
Cursors byLoc.size(): 0

replication
master: 0
slave:  0
initialSyncCompleted: 1

DBTOP  (occurences|percent of elapsed)
NS	total	Reads	Writes	Queries	GetMores	Inserts	Updates	Removes
GLOBAL	1	0.00%	1	0.00%	0	0.00%	1	0.00%	0	0.00%	0	0.00%	0	0.00%	0	0.00%
olympics.olympic_host_city	1	0.00%	1	0.00%	0	0.00%	1	0.00%	0	0.00%	0	0.00%	0	0.00%	0	0.00%

It was interesting to see a whatsmyuri command. Will need to investigate that further.