Hi.
I think you can mix Standard and Enterprise licences on a farm, based on who accesses what. For instance, if you have a site accessed by 20 people who use Excel Calc Server, they will need Enterprise. If you have another 80 people who access a different site with no BDC, ECS, InfoPath or BI tools activated (this is easy to do) then you need an additional 80 standard CAL's. Both sites can run from the same server in the same farm (which obviously has an Enterprise key associated with it).
From this page: http://office.microsoft.com/en-us/sharepointserver/HA101655351033.aspx#2
What if some users need enterprise features and others do not?
Once the enterprise features of Office SharePoint Server 2007 are enabled, every client accessing enterprise functionality on that server, or servers in a farm, is required to have an Enterprise CAL in addition to their standard client access.
In addition, you can now use the same servers on a common farm to serve up an internet site, extranet site and an Intranet site. You can even use the MOSSFIS licence to provide access to people within your organisation provided the same content is available to people external to your organisation. This is the premise behind an extranet site, but if your 100 000 user company is partnering or IP sharing with a 20 person company, then a MOSSFIS licence per server will cover all required CAL's. This change came in late 2008 from memory (mainly because people were spitting chips that they had to buy 2 server farms for different content).
From the same page...
Can I mix SharePoint editions in the same farm?
For Microsoft Office SharePoint Server and Microsoft Office SharePoint Server for Internet Sites, the use of a server determines its licensing requirements, not its location in one farm or another. Consequently, you can mix Microsoft Office SharePoint Server and Microsoft Office SharePoint Server for Internet Sites within the same farm--so long as the use cases are licensed appropriately. However, because Microsoft Office SharePoint Server 2007 for Internet Sites licenses the functionality of the Microsoft Office SharePoint Server Enterprise CAL, you will need to ensure that your users only access functionality they are licensed for based on their CALs.
Microsoft Office Forms Server and Microsoft Office Forms Server for Internet Sites cannot be mixed with Microsoft Office SharePoint Server and Microsoft Office SharePoint Server for Internet Sites.
Additionally, as an accommodation for various possible deployment scenarios, customers wishing to consolidate their SharePoint needs under a single deployment may acquire licenses for both Office SharePoint Sever and Office SharePoint Server for Internet Sites, assign those licenses to the same servers, and use the same running instances of the software simultaneously under both licenses. However, customers must acquire CALs as required under the Office SharePoint Server use rights for users and devices accessing content in any manner not permitted under the Office SharePoint Server for Internet sites use rights.