I calculated the answer by starting with Gavin's "bitcointools" and modifying it to track account balances.
Here's what I found. Each line shows the number of addresses after the specified block was found, at the end of each month. So the 1st line is saying that after block 2543 was found at the end of Jan 2009, there were 2,439 funded addresses, and all 2,569 were 'active' in January 2009. The 'active' number is the number of addresses involved in a transaction in the preceding month, and can be bigger than the number of addresses which remain funded at the end of that month.
-block --date-- --total-- ---used--%total- -active---%total---%used-
2543 Jan 2009 2,569 2,439 (94.94%) 2,569 100.00% 105.33%
5923 Feb 2009 5,975 5,646 (94.49%) 3,447 57.69% 61.05%
9389 Mar 2009 9,444 8,868 (93.90%) 3,486 36.91% 39.31%
12831 Apr 2009 12,893 12,125 (94.04%) 3,495 27.11% 28.82%
16214 May 2009 16,288 15,388 (94.47%) 3,424 21.02% 22.25%
18450 Jun 2009 18,528 17,543 (94.68%) 2,267 12.24% 12.92%
20375 Jul 2009 20,456 19,347 (94.58%) 1,973 9.65% 10.20%
21939 Aug 2009 22,021 20,749 (94.22%) 1,639 7.44% 7.90%
24098 Sep 2009 24,180 22,647 (93.66%) 2,212 9.15% 9.77%
26224 Oct 2009 26,311 24,374 (92.64%) 2,231 8.48% 9.15%
28441 Nov 2009 28,533 26,411 (92.56%) 2,262 7.93% 8.56%
32489 Dec 2009 32,611 29,827 (91.46%) 4,420 13.55% 14.82%
-block --date-- --total-- ---used--%total- -active---%total---%used-
37493 Jan 2010 37,647 34,342 (91.22%) 5,304 14.09% 15.44%
43096 Feb 2010 43,329 37,274 (86.03%) 7,009 16.18% 18.80%
48297 Mar 2010 48,615 40,329 (82.96%) 6,308 12.98% 15.64%
53875 Apr 2010 57,806 42,688 (73.85%) 10,839 18.75% 25.39%
58815 May 2010 63,368 44,620 (70.41%) 6,762 10.67% 15.15%
63561 Jun 2010 69,540 47,011 (67.60%) 7,056 10.15% 15.01%
71436 Jul 2010 94,992 53,227 (56.03%) 27,957 29.43% 52.52%
77452 Aug 2010 107,172 55,995 (52.25%) 14,162 13.21% 25.29%
82997 Sep 2010 120,562 56,490 (46.86%) 16,883 14.00% 29.89%
88892 Oct 2010 135,952 58,428 (42.98%) 18,921 13.92% 32.38%
94801 Nov 2010 155,639 60,615 (38.95%) 22,307 14.33% 36.80%
100409 Dec 2010 176,326 65,622 (37.22%) 23,377 13.26% 35.62%
-block --date-- --total-- ---used--%total- -active---%total---%used-
105570 Jan 2011 210,243 69,030 (32.83%) 37,989 18.07% 55.03%
111136 Feb 2011 267,542 78,813 (29.46%) 62,381 23.32% 79.15%
116038 Mar 2011 353,093 90,304 (25.58%) 92,321 26.15% 102.23%
121126 Apr 2011 443,112 99,090 (22.36%) 99,144 22.37% 100.05%
127865 May 2011 631,480 132,152 (20.93%) 203,052 32.15% 153.65%
134121 Jun 2011 1,061,590 196,975 (18.55%) 462,886 43.60% 235.00%
139035 Jul 2011 1,548,806 380,056 (24.54%) 531,002 34.28% 139.72%
143408 Aug 2011 1,930,979 487,833 (25.26%) 428,095 22.17% 87.75%
147565 Sep 2011 2,205,926 533,670 (24.19%) 318,294 14.43% 59.64%
151314 Oct 2011 2,411,298 546,269 (22.65%) 241,105 10.00% 44.14%
155451 Nov 2011 2,601,999 553,251 (21.26%) 225,012 8.65% 40.67%
160036 Dec 2011 2,768,500 568,177 (20.52%) 199,879 7.22% 35.18%
-block --date-- --total-- ---used--%total- -active---%total---%used-
164780 Jan 2012 2,956,869 580,453 (19.63%) 236,927 8.01% 40.82%
165643 6 Feb 12 2,994,694 583,126 (19.47%) 62,300 2.08% 10.68%
So the number of non-empty addresses has been around 600,000 since September 2011.
As of block 165643, the ten most common values for addresses to store were:
0.00000000 (in 2411568 addresses)
50.00000000 (in 40631 addresses)
0.00000001 (in 35921 addresses)
0.05000000 (in 22806 addresses)
0.00100000 (in 22439 addresses)
0.02000000 (in 18160 addresses)
0.01000000 (in 13178 addresses)
1.00000000 (in 11636 addresses)
0.00500000 (in 9502 addresses)
0.00000002 (in 7628 addresses)
The next 43 most common values after that were the 43 values from 3 satoshis to 45 satoshis, with between 5472 and 5799 addresses having each value.
I don't know what's special about 0.00000045 and not 0.00000046, but only one other value with a non-zero 8th decimal digit occurred in more than 25 addresses. That was 0.00000703, which occurred in 101 different addresses.
I also found a bunch of weird looking addresses, which nobody will ever be able to spend from. http://blockchain.info/address/871a40e5e61b96b6171f1b435788082edadda7a8 shows a lot of transactions sending tiny amounts to human-readable addresses, spelling out paragraphs of text.
As of block 165643, from Mon Feb 6 2012:
2 addresses have 6 or more figures (100,000 <= x):
105,258.88 12WmWuSQCgpEkPQTZ4ABijQaz6TCV69ZpE and
105,555.00 1933phfhK3ZgFQNLGSDXvqCn32k2buXY8a
70 addresses have 5 figures (10,000 <= x < 100,000)
740 addresses have 4 figures (1,000 <= x < 10,000)
6,552 addresses have 3 figures (100 <= x < 1,000)
66,118 addresses have 2 figures (10 <= x < 100)
50,638 addresses have 1 figure (1 <= x < 10)
459,006 addresses have a fraction (0 < x < 1)
2,411,568 addresses have 0 BTC
