You can't search lists or libraries in MOSS 2007. Or some older lists return results but nothing new. Essentially "This List:" doesn't return results while searching, but "This Site:" does return results.
There are a couple of possibilities here for the cause and I've read some ridiculous theories on this. Most of them point as your SSP being corrupt and you need to create it. Or you've corrupted your databases and need to restore them.
Really the first place I thought to check was Authentication. I found that there 2 different types of content in Sharepoint / MOSS 2007. Long story short, you can search the .aspx pages, separately from List and Library items. I've found this to be reflected in your indexes also.
What actually triggered this for me was resetting SPN's for Sharepoint. I found that some of the SPN's were throwing errors on the Domain controllers so I deleted and recreated them. In another case I changed from Kerberos to NTLM authentication to resolve some immediate issues but didn't update my search service account.
Fixing "This List" search issues:
1) Login to your SSP
2) Under Search, select Search Settings.
You will see the following:
Indexing status: "if you're in error it will probably say something other than IDLE"
Items in index: "# of items"
Errors in log: "# of errors"
Content sources: "Local Office SharePoint Server sites"
Crawl rules: "will be different depending on what you've defined"
Default content access account: "Should be a Domain account
Managed properties: Default is 130 defined properties
Search alerts status: "Alert status"
Propagation status: Idle
3) Click on on "Manage Crawl Rules"
Note: You should have the URL of your Site configured here.
4) Click on the URL of the site with the issue(s).
5) Select "Include all items in this path" and check Crawl complex URLs (URLs that contain a question mark (?)).
6) Then select "Specify a different content access account"
7) Enter in the Search User account and password.
8) Use the Breadcrumb and select "Search Settings".
9) Click on "Content Sources and Crawl Schedules".
10) Select your content source ("Local Office SharePoint Server Sites" is the default).
11) Make sure that the URL for your SharePoint site is listed in the "Type Start Address Below" box.
12) Under Crawl Settings, select "Crawl everything under the hostname for each start address".
13) Then set your crawl schedules. Usually you do a Full Crawl outside of your business day and hourly for your incremental crawls.
14) Check the box for "Start Full Crawl"
You can monitory the status of the crawl at the "Search Settings" page. Depending on how much content you have, will determine how long it will take to index. Also it will go through and delete old indexes for deleted content at the sametime. This will add time for the full crawl to complete.