We had our first automatic adjustment of the proof-of-work difficulty on 30 Dec 2009.
The minimum difficulty is 32 zero bits, so even if only one person was running a node, the difficulty doesn't get any easier than that. For most of last year, we were hovering below the minimum. On 30 Dec we broke above it and the algorithm adjusted to more difficulty. It's been getting more difficult at each adjustment since then.
The adjustment on 04 Feb took it up from 1.34 times last year's difficulty to 1.82 times more difficult than last year. That means you generate only 55% as many coins for the same amount of work.
The difficulty adjusts proportionally to the total effort across the network. If the number of nodes doubles, the difficulty will also double, returning the total generated to the target rate.
For those technically inclined, the proof-of-work difficulty can be seen by searching on "target:" in debug.log. It's a 256-bit unsigned hex number, which the SHA-256 value has to be less than to successfully generate a block. It gets adjusted every 2016 blocks, typically two weeks. That's when it prints "GetNextWorkRequired RETARGET" in debug.log.
minimum 00000000ffff0000000000000000000000000000000000000000000000000000
30/12/2009 00000000d86a0000000000000000000000000000000000000000000000000000
11/01/2010 00000000c4280000000000000000000000000000000000000000000000000000
25/01/2010 00000000be710000000000000000000000000000000000000000000000000000
04/02/2010 000000008cc30000000000000000000000000000000000000000000000000000
14/02/2010 0000000065465700000000000000000000000000000000000000000000000000
24/02/2010 0000000043b3e500000000000000000000000000000000000000000000000000
08/03/2010 00000000387f6f00000000000000000000000000000000000000000000000000
21/03/2010 0000000038137500000000000000000000000000000000000000000000000000
01/04/2010 000000002a111500000000000000000000000000000000000000000000000000
12/04/2010 0000000020bca700000000000000000000000000000000000000000000000000
21/04/2010 0000000016546f00000000000000000000000000000000000000000000000000
04/05/2010 0000000013ec5300000000000000000000000000000000000000000000000000
19/05/2010 00000000159c2400000000000000000000000000000000000000000000000000
29/05/2010 000000000f675c00000000000000000000000000000000000000000000000000
11/06/2010 000000000eba6400000000000000000000000000000000000000000000000000
24/06/2010 000000000d314200000000000000000000000000000000000000000000000000
06/07/2010 000000000ae49300000000000000000000000000000000000000000000000000
13/07/2010 0000000005a3f400000000000000000000000000000000000000000000000000
16/07/2010 000000000168fd00000000000000000000000000000000000000000000000000
27/07/2010 00000000010c5a00000000000000000000000000000000000000000000000000
05/08/2010 0000000000ba1800000000000000000000000000000000000000000000000000
15/08/2010 0000000000800e00000000000000000000000000000000000000000000000000
26/08/2010 0000000000692000000000000000000000000000000000000000000000000000
date, difficulty factor, % change
2009 1.00
30/12/2009 1.18 +18%
11/01/2010 1.31 +11%
25/01/2010 1.34 +2%
04/02/2010 1.82 +36%
14/02/2010 2.53 +39%
24/02/2010 3.78 +49%
08/03/2010 4.53 +20%
21/03/2010 4.57 +9%
01/04/2010 6.09 +33%
12/04/2010 7.82 +28%
21/04/2010 11.46 +47%
04/05/2010 12.85 +12%
19/05/2010 11.85 -8%
29/05/2010 16.62 +40%
11/06/2010 17.38 +5%
24/06/2010 19.41 +12%
06/07/2010 23.50 +21%
13/07/2010 45.38 +93%
16/07/2010 181.54 +300%
27/07/2010 244.21 +35%
05/08/2010 352.17 +44%
15/08/2010 511.77 +45%
26/08/2010 623.39 +22%