There is capacity planning guidance–very detailed guidance!–on TechNet. Capacity is really about performance–there are very few hard-wired limits–and performance is based on usage and content.
Here are the software boundaries and capacity planning thresholds and limits. http://technet.microsoft.com/en-us/library/cc262787.aspx. Short answer: SharePoint 2010 scales further (way further) and more flexibly than ever before
Some guidelines include: 100GB per site collection, 200GB per content database (unless you have single-site document repositories, in which case TBs are supported), 300 content DBs per web app, 250,000 Web sites per site collection (2,000 under any one site), and tens of millions of items/documents per list/library. But again, these are HIGHLY dependent upon hardware, topology, usage, etc.
There is no hard limit on the number of items in a list. A good rule of thumb is 5,000. 5,000 are the new 2,000 🙂
Oh sorry–LIST size limit is tens of millions of items. However you can manage the performance of lists (Spencer’s note about 5,000) in several ways. It’s all about query (and view)… THOSE are the limits… the list itself can have tens of millions of items.
The feature we’re talking about is List Throttling, which allows an admin to control the impact of large lists on the rest of the farm.
<i>This question was answered by Spencer Harbar, Enterprise Architect at Microsoft and Dan Holme, Consultant and Trainer, Intellium.</i>