I work for a large local government organisation who are about to embark on using SharePoint to replace our ageing intranet with an all-singing all-dancing collaborative sit
The main problem of using Microsoft Office SharePoint Server for your public web site is the very high licensing cost. You will require something like an Office SharePoint Server 2007 for Internet Sites license. The price is listed as USD 41,134 on the Microsoft pricing page. Other than that I have found that SharePoint is very feature rich and certainly suited for web sites other than the "standard intranet".
My company has started testing Sharepoint Services (WSS 3.0) as it is a free version, it is short some features of MOSS (Sharepoint 2007) but it is free and comes with much of the things a company may require. However it is a Microsoft product so its always got some licensing 'gotchas'.
WSS has worked fine for us internally with document management, team collaboration, wiki / blog type functionality, it integrates with LDAP / Windows authentication (it is MS after all).
Dont' forget to factor in user acceptance into your decision. Having to train on two different systems can quickly become a PITA if there are a number of people updating the site.
Not knowing anything about the UmbracoCMS, i would say that using a sharepoint system will allow you to do the things you need to do now as well as extend that functionality into the future. As an additional comment to those made above, keep in mind that you will want some kind of publishing feature for this external site. WSS and MOSS can accomadate the feature. Migration of content is also interesting since you can develope functionality on your current farm and then selectivly push these features onto the external farm.
To sum up: Though the setup is still a pain in the butt, you will have a much better administrative experience if your topology is homogenous. After all, this is what you current problem is anyway... why create more chaos?
Without much deep knowledge of sharepoint (only used it for internal sites), I can only help out on the Umbraco part. As a developer I love the control I have over the xhtml output, no surprises and extremley flexible. Powering large sites such as Conde Nast sites outside USA, Heinz, Hasselblad + many more means that it does scale. But, I think the biggest asset is the community, it's so easy to get great help should you encounter any problems or if you have a question. See http://our.umbraco.org. I can also recommend the videos, I think it's a very nice way to learn the basics combined with the Creative Website Starter kit by Warren Buckley (available from the package repository).
Many public organizations use SharePoint for publicly-facing internet and collaboration sites. When architected, designed, and implemented to your specifications, SharePoint can meet the requirements you have described (including the accessibility requirements).
The real gotchas with SharePoint (as with any major software engineering project) are that you have a team of competent professionals who know what they are doing.
Here are some links to lists of sites that use SharePoint to get a better idea of how existing organizations are using SharePoint: