I write a lot of applications in access, and use SQL server as the back end for nearly all of them. Our SQL server is not really that heavily used though, if it was I would consider leaving some access - only. What I really hate about access is the jet engine. I have written some queries that won't run in access (usually involving different calculations using data from multiple tables), but if I put the exact same tables in SQL Server, link to them, and run the query that way they work fine. This frustrates me to no end!
If you are adding 1 million rows per year, and your tables are even a little wide, I'd think you would outgrow access rather quickly. I am looking at an access db sent by a client right now for some data overlays and analysis that is 1.6 gb (performance is ATROCIOUS!). By my rough estimation, this has 5-6 million rows in it (3.2 million in the main table). So, as a short term solution, access will probably work for you. If you want something more dependable over the long term, go with SQL Server.
Hope this helps,
Alex
Ignorance of certain subjects is a great part of wisdom