This has been happening a lot to me recently:

[2013-05-06 12:30:02] Accepted 46d12787 Diff 3/1 GPU 0 pool 0
[2013-05-06 12:30:05] Pool 0 http://pool.50btc.com:8332 not responding!
[2013-05-06 12:30:05] Switching to pool 1 http://stratum.btcguild.com:3333
[2013-05-06 12:30:20] Accepted 34957ca4 Diff 4/1 GPU 0 pool 1
[2013-05-06 12:30:25] Accepted f6233d52 Diff 1/1 GPU 0 pool 1
[2013-05-06 12:30:28] Accepted 398a11d3 Diff 4/1 GPU 0 pool 1
[2013-05-06 12:30:44] Accepted bf6d1a6e Diff 1/1 GPU 0 pool 1
[2013-05-06 12:30:46] Accepted aff54d77 Diff 1/1 GPU 0 pool 1
[2013-05-06 12:30:55] Accepted ae11e4ba Diff 1/1 GPU 0 pool 1
[2013-05-06 12:30:57] Pool 0 http://pool.50btc.com:8332 alive
[2013-05-06 12:30:57] Switching to pool 0 http://pool.50btc.com:8332
[2013-05-06 12:31:00] Accepted ad03956f Diff 1/1 GPU 0 pool 0
[2013-05-06 12:31:01] Accepted 5029ed46 Diff 3/1 GPU 0 pool 0

I have 4 miners on 4 different computers, and they each experience this.  The times above are in US/Central. (most recent failure on a 680 mhash miner)

I only do about 2GH/s combined, and I reset my stats on BTC guild last night about 12 hours ago, and since then, I've failed over to them enough to submit 895 shares.

I saw a post a few days back on the previous page as well on this same topic.