As long as we're talking about server-based computing and how Citrix fits into the market, we have to address the million dollar question: With all of Microsoft's advancements in Terminal Services for Windows Server 2008, will you even need Citrix in the future? When you migrate to 2008, can you run pure Terminal Services?
This is a question that people have been arguing about for ten years. Remember that in 1997, when you bought "Citrix," you also bought "Windows." The Citrix WinFrame product was a stack of about 40 floppy disks that contained the complete Windows NT 3.51-based OS as well as the Citrix server-based computing extensions.
Then in 1998, Microsoft decided that instead of licensing the NT 4.0 code to Citrix, Microsoft would just build their own SBC product. Citrix's future looked bleak. Luckily some persistent folks at Citrix convinced Microsoft to license the core "multi-user" source code from them instead of writing it on their own. Microsoft agreed, and it was decided that Microsoft would sell the "core" multi-user functionality of Windows, and Citrix would sell the "add-on" product that allowed people to really use SBC in the real world.
That was 1998. Microsoft sold Windows NT Server 4.0, Terminal Server Edition, and Citrix sold MetaFrame 1.0 and then MetaFrame 1.8. Microsoft Windows included the base multi-user kernel and allowed for very basic connections, and Citrix MetaFrame added support for seamless windows, connections from non-Windows client devices, support for more colors and higher resolution, clipboard mapping, printing, etc.
Then for Windows 2000, Microsoft announced that Terminal Server would be "built in" to Windows, and that it would be enhanced to offer features like more colors, higher resolution, and clipboard mapping. There were no bloggers in those days, but if there were, they'd have been up in arms screaming, "Microsoft will kill Citrix!"
Of course Microsoft didn't kill Citrix, and Citrix continued to out-innovate Microsoft. By the time Windows Server 2003 rolled around, Microsoft added universal printing and SSL encryption to the core Windows Terminal Server feature set, but in these days, Citrix's leading features were things like Web Interface, an SSL gateway, intelligent load balancing, and application publishing. Again, there was no danger that Microsoft would kill Citrix, because Citrix had a richer feature set.
(As a side note, through all this it's important to remember that for every license of Citrix Presentation Server that's sold, the customer also needs a Terminal Server user CAL from Microsoft. Therefore if Citrix was selling $500m worth of Presentation Server licenses in those days, Microsoft was probably getting $300m in "free" TS CAL sales. So Microsoft certainly had no reason to want to take out Citrix.)
Shortly after Microsoft released Windows Server 2003, rumors started flying that they were working on something called "Project Bear Paw." Bear Paw was supposedly a set of updates for Terminal Server that would give it functionality very near Citrix Presentation Server.