InfoPath Forms Services are part of Enterprise edition.

Here you have nice overview

 - [Compare SharePoint Editions][1]

Also take a look at my answer on licensing requirements for InfoPath forms:

 - http://sharepoint.stackexchange.com/questions/17022/sharepoint-2010-standard-and-forms/17025#17025

  [1]: http://sharepoint.microsoft.com/en-us/buy/Pages/Editions-Comparison.aspx