So I have installed SharePoint 2010, went to some trainings from MS, read blogs, carried out a few POCs , examined the architecture and after all this thought to share my thoughts on how people familiar with the 2007 version should consider in designing a SharePoint 2010 solution.
First of all, the obvious; familiarize yourself with the tool as much as possible. Although this may seem common sense, you’d be surprised as to the number of implementations out that have coded in some feature of SharePoint that’s already available. I don’t really blame the people who did it, since there was a lot of documents on SharePoint that gradually evolved, so it is possible that once the design was done and the project is nearly ready for production, some documentation crops up that has an impact on the design. (This actually happened to me, however luckily there was not much of impact. I actually found out after the deployment, when I reread the MSDN documentation and found my understanding on one aspect was wrong. I realized the documentation was changed after some more searching and reaching a blog that had a copy/paste of the text from an earlier version of the same page).
SharePoint 2010 has lots more features than the earlier version (have listed some of the differences here), hence that means a lot more preparation before designing your first SharePoint 2010 project.
From an architectural standpoint, a SharePoint implementation can be split across multiple farms, since services and even content types can be shared across multiple farms, so you need to figure out which services will be used most, where to host your services, plan for future growth etc. You also need to know that some features may be adopted pretty quickly – eg. the Office Web Apps and Search, so you may need to plan splitting out the services into different machines. On the subject of search, unlike the previous versions, it is possible to split out or replicate the index, so thats something additional that you need to decide.
As per all the info from MS, the boundaries have been extended; however its still not infinity :). Do make sure you read though the 2010 capacity planning document before deciding on the architecture your solution.