One of the major deficiencies in SharePoint 2007 was the difficulty involved in making sites comply with accessibility standards.

SharePoint 2010 has made significant improvements in this area including standards compliance, keyboard support, tab support, reduced use of tables etc.

There is a useful article on some of the specific improvements that can be found on http://blogs.msdn.com/sharepoint/archive/2010/03/09/accessibility-and-sharepoint-2010.aspx

I'm sure there will be specific areas that still require some customisation to get to the level of support expected for accessibility standards but the amount of work to acheive this will be significantly reduced!

Today, Microsoft and HiSoftware are pleased to announce the release of the Accessibility Kit for SharePoint (AKS) v2.0.  The AKS v2.0 provides an accessible development framework and significantly reduces the level of effort and knowledge needed by customers and partners to improve the accessibility of SharePoint based sites and applications.

See http://blogs.msdn.com/sharepoint/archive/2009/03/09/announcing-the-accessibility-kit-for-sharepoint-aks-v2-0.aspx for further details... 

A common requirement when undertaking interface design for Sharepoint is to make the interface accessible (accessible meaning that users with visual or other impairment).

In the UK, the Disability Discrimination Act implicitly states that an intranet should be accessible to all employees, so even your private intranet should be capable of coping with user impairments.

The Sharepoint Team issued a statement a while back (pre release) about the accesibilty capabilites of Sharepoint

Customising the Sharepoint interface to make this work can be a real uphill task. The interface uses a lot of nested tables making relative font sizing difficult as noted by Diantha. The Sharepoint accessibility kit can be your friend here though.

An example of the kits usage is show below - a resizing tool to change absolute to relative sizing

But beware, this does not 'magically' fix your accessibility problems and you still need to consider the interface requirements early on.

The kit also includes a set of more accessible style sheets that can be applied

Another great resource for Sharepoint design, including accessibility is Heather Solomon