I was just trying to install Apache Directory Studio 1.5.3 from within Eclipse 3.7. It's saying that the certificate signing the software (or maybe the update site) is both self-signed and expired in January 2010. This is a bit more worrying than even having no certificate, since the 1.5.3 release is from April 2010, and I'm kind of puzzled that it was signed with a certificate that was already several months out of date when the release was made, in addition to being self-signed. I'm also trying this more than a year after the 1.5.3 release occurred, so the fact that the situation remains as I've described is quite worrying from the perspective of having security issues noticed and addressed in a timely fashion.
There are many valid ways to handle the issue of code signing, including deciding that it's not useful security to do it at all, making an Apache-specific certificate authority, or paying for a commercial certificate as is done for the *.apache.org HTTPS web sites. The current situation with the Eclipse update site encourages false guarantees of security and, if Apache's users are taught to ignore such warnings, exposes them to man-in-the-middle or other malicious attacks when they think they are being protected by the security reputation of the Apache Software Foundation.
The time estimate I have given is assuming you decide to generate some new certificate by whatever commercial or non-commercial method, and may include the time to deal with a vendor and/or rebuild the software. If you simply decide to switch your repository to unsigned, my estimate will probably be too large.