SpecFlow 1.5 comes with Intellisense for step-completion in VisualStudio 2010:
When you are writing feature files, you get Intellisense after Given/When/Then/And/But keywords.
As usual you can trigger Intellisense with Ctrl-Space.
The feature is switched off by default. You have to enable it under Menu –> Tools –> Options then choose SpecFlow. In the Editor Settings set 'Enable Intellisense' to True: