Lookup across site collection

Out of box not possible
Third party available..

How to exclude items from a crawl based on items properties

custom creator can be created to fulfill the request

Edit in datasheet does not work with webpart filter

create seperate dataview and then datasheet will work

progmatically setting home page of publishing site

 Publishing sites to set the Home Page.
you can set SPWeb.RootFolder.WelcomePage

User deleted and re-added to AD - Profile not found

You need a temp account on which you migrate the profile first then to the id.

how to update users email if there is no interface in UI

Dont go to database. instead use the power shell to do same

Add-PSSnapin Microsoft.SharePoint.PowerShell
$site = Get-SPSite http://example.com
$web = $site.RootWeb
$user = $web.SiteUsers.GetByEmail("usr@example.com")
$user.Email = "user@example.com"
$user.Update()

sharepoint 2010 sesion state

1. Enter the following PowerShell command in the SharePoint 2010 Management Shell window:

Enable-SPSessionStateService –DefaultProvision

1. On each web application for which you want to use session state, edit the web.config file and set the enableSessionState property of the pages element as follows:

How to overwrite documents that require check out?

you have to fist check out file

foreach ($file in $files) {
    $file.CheckOut()
    $stream = $file.OpenRead()
    $done = $list.RootFolder.SubFolders["Hertel"].Files.Add($file.Name, $stream, $true)
    Write-Host $done.Name "Uploaded" -BackgroundColor Green
}

Click to Copy List Item to New in same list

Manually can be done by editing in datasheet
 designer work flow also works http://www.wonderlaura.com/Lists/Posts/Post.aspx?ID=107

Will exceeding 2000 documents in a SharePoint 2007 Library folder degrade only the performance of that library?

2000 is the view threshold. Index can be added