Chalk this down as the 1045th* time I've been reminded that you learn something new about SharePoint every day.
*The number of days I've been working with SharePoint... :)
Turns out there's a setting at the IIS level that will block list attachment uploads above 50MB. Below 50MB, any max upload size you pick (for example, 10MB) works fine for both list attachments and document uploads and is enforced directly through web app settings. Above 50MB, you need to make at least one minor tweak to your web.config to allow larger uploads.
Complete solution for max upload sizes over 50MB:
if you're using SQL storage for your SharePoint content, the maximum useful value for the maxRequestLength should be 2097152 (2GB in kilobytes = 2 * 1024 * 1024). Keep in mind that other limits may affect large uploads, most notably:
For more on the httpRuntime web.config element, see http://msdn2.microsoft.com/en-us/library/e1f13641(VS.80).aspx .