When creating a NON-Unique index, index is created normally. On a collection with 3.5M documents:
I did a db.<dname>.ensureIndex(

):

Fri Nov 13 13:44:24 cpu: 4000 0 0%
Fri Nov 13 13:44:25 building new index on

for <dbname>.Farm...
Fri Nov 13 13:44:25 Buildindex <dbname>,Farm idxNo:1 { ns: "<dbname>.Farm", key:

, name: "uid_1" }
Fri Nov 13 13:48:22 done for 3517687 records 237.078secs
Fri Nov 13 13:44:25 insert <dnbame>.system.indexes 237259ms
Fri Nov 13 13:48:22 cpu: 238225 237265 99%

I did a db.<dbname>.dropIndexes();
{
"nIndexesWas" : 2,
"msg" : "all indexes deleted for collection",
"ok" : 1
}

Then db.<dname>.ensureIndex(

,

)

Fri Nov 13 13:51:10 cpu: 4000 0 0%
Fri Nov 13 13:51:10 building new index on

for <dname>.Farm...
Fri Nov 13 13:51:11 Buildindex <dbname>.Farm idxNo:1 { ns: "<dname>.Farm", key:

, name: "uid_1", unique: true }
Fri Nov 13 14:05:28 User Exception E11000 duplicate key errorindex: <dname>.Farm.$uid_1 dup key: { : 501272713 }
Fri Nov 13 14:05:38 <dbname>.system.indexes Caught Assertion insert, continuing
Fri Nov 13 13:51:10 insert <dbname>.system.indexes exception userassert:E11000 duplicate key errorindex: <dbname>.Farm.$uid_1 dup key: { : 501272713 } 867252ms
Fri Nov 13 14:05:38 cpu: 867851 867252 99%

During this second run, mongo went almost 2GB into swap and as you can see, took almost 4 times as long before even detecting a conflict. I had to restart mongodb to clear up the ram usage.. it was using all system ram (RES 7.2G). The whole collection size is around 16G.

> db.<dbname>.dropIndexes();
{
"nIndexesWas" : 1,
"msg" : "all indexes deleted for collection",
"ok" : 1
}

Now, even adding a regular index causes massive swap use:
db.<dbname>.ensureIndex(

)

top - 14:16:50 up 20:28, 4 users, load average: 1.50, 0.89, 0.93
Tasks: 66 total, 2 running, 64 sleeping, 0 stopped, 0 zombie
Cpu(s): 42.7%us, 0.3%sy, 0.0%ni, 49.3%id, 0.7%wa, 0.0%hi, 0.0%si, 6.9%st
Mem: 7872040k total, 7855940k used, 16100k free, 104k buffers
Swap: 8388600k total, 1092068k used, 7296532k free, 5337924k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
5480 mongo 19 0 22.1g 7.3g 5.1g S 82 96.6 0:13.91 mongod

It completes, but ram is not freed:

Fri Nov 13 14:13:34 cpu: 4000 0 0%
Fri Nov 13 14:13:34 building new index on

for <dbname>.Farm...
Fri Nov 13 14:13:34 Buildindex <dbname>.Farm idxNo:1 { ns: "<dbname>.Farm", key:

, name: "uid_1" }
Fri Nov 13 14:16:58 done for 3517687 records 203.592secs
Fri Nov 13 14:13:34 insert <dbname>.system.indexes 203648ms
Fri Nov 13 14:16:58 cpu: 204073 203655 99%