Maybe, for two reasons.

1) Short-term losses can be deducted from short-term gains and long-term gains. Let's say your ETF lost $1000 in the last 364 days. If you have more than $1000 in realized short-term gains (gains from stocks that you have already sold), then you could save $300 in the 30% bracket. If you have no short-term gains but more than $1000 in realized long-term gains, then you could save $150. If you have no realized capital gains, you can deduct $1000 of capital losses from your income, in this case saving you $300. If you lost more than $3000, losses beyond $3000 can't be deducted, but can be carried forward to the next year.

2. If you buy the same ETF back tomorrow, it's called a "wash sale," and the loss cannot be deducted. This is defined in more detail here:
http://www.investopedia.com/articles/04/122704.asp
It may be possible to buy back a different ETF containing similar securities; this is a gray area in the law.