When Slashdot links a site, often a lot of readers will hit the link to read the story or see the purty pictures. This can easily throw thousands of hits at the site in minutes. Most of the time, large professional websites have no problem with this, but often a site we link will be a smaller site, used to getting only a few thousand hits a day. When all those Slashdot readers start crashing the party, it can saturate the site completely, causing the site to buckle under the strain. When this happens, the site is said to be "Slashdotted."
This recently happened to my humble personal site when the Hall of Technical Documentation Weirdness was posted to Slashdot. In one day, I had over 60,000 visitors to my site, which brought it to its virtual knees for a few hours. The always witty Tom Murphy links to an interview with Slashdot's founder which discusses this very issue (among other things).
As Tom alludes to, Slashdot is very skeptical about 'meekly veiled attempts at ghosted PR postings'. It does present an interesting public relations challenge. For clients and myself, I've submitted about ten 'news items' to Slashdot, and had them accepted twice. Once was the aforementioned Hall and the other was a story about NewBay Software. Mine is a pretty good record, considering that they get between 300 and 600 submissions per day, and publish maybe 20.