The first week of January, when we wrote about the Major Google Search Results Changes, January 2009, I noticed the Google Sitelinks for this site, disappeared. Today, I noticed they are back. Today, also, many webmasters are noticing they have lost their sitelinks.
A WebmasterWorld thread has two webmasters noting that their Sitelinks disappeared both in the Google search results and in Google Webmaster Tools. They both are concerned and asked if they should be.
One noted his/her traffic is up 30% month over month and rankings seem to be steady. But still, he/she is concerned.
Personally, I was a bit taken back - but I did nothing. I decided to see what would happen with my traffic. I just checked my traffic now, and it is up from a week ago, but it is fairly level from a month ago. See, I forgot to even look to see if those Sitelinks had an impact and now they are back. But the smart thing would be to check in and see the impact on traffic and then maybe even rankings. I would be concerned until you see a negative impact over on that site. There can be many reasons why Google would temporarily drop the sitelinks for your site. Many of those reasons might be an issue on Google's side.
I agree with Tedster's comment in the thread:
Seeing Google traffic up by 30% is a very good thing. I would not have major concern about the loss of sitelinks if search traffic goes up (especially well-targeted search traffc), but certainly do keep a watch on the situation. It sounds rather odd, especially that potential sitelinks don't show in WMT. If you haven't made structural changes to your site, this might well be a Google bug.
Forum discussion at WebmasterWorld.