Whenever a user has requested an alert for a list or another object i Sharepoint there is a default delay of 10 minutes. Users are puzzled by not having received an alert when other users tell them that updates have been made to a list or document library.
These settings are not (as far as I know) available from the Central Administration Pages, so the command line is the way to go to speed things up [from Sharepoint University]:
stsadm.exe -o setproperty -pn job-immediate-notification -pv 3
The -pv [PropertyValue] part sets the delay in minutes.
UPDATE: Setting this property seems to make sharepoint push out all alerts ever sent again. When I did this in the production environment every user that had an alert set up for a list got a separate alert for every list item. Fortunately the portal had only been in production for a couple of weeks, so it affected few users, but you should definately investigate this before executing the command!