When you see that you have zero opens for a campaign, first check and make sure that you have tracking selected for that campaign. Look on the report page for "tracking disabled" or "tracking enabled" noted by going to Reports > Subscriber Activity > Trends.
In HTML campaigns, when you check the box to "Track Opens", MailChimp embeds a tiny, invisible graphic to the bottom of your HTML email. This "open tracker" graphic (sometimes referred to as a "web beacon") is unique to each campaign you send.
Since we cannot add code to do this in your plain text emails (we send only text for that version - not any images), we track the number of clicks in your campaign to determine opens. If you selected to track opens on your plain text campaign and there were no links for your recipients to click on, there would be nothing to track. We would show zero opens in your report. We do not do tracking on system tags like *|UNSUB|* or *|UPDATE_PROFILE|*.
In rare cases, something in your incoming filters will block our tracking image. Check out this link for more info www.answers.com/topic/web-bug-1.