How To Handle Corporate Custom VBA Code for Excel

I'll try not to be verbose but this is something that I find hard to articulate without being verbose so let me provide the scenario that brought about the question.

In our company we have a number of Excel users most of which are on either 2007 or 2010 with most moving to 2010 in the next few months. We have gotten requests from some executives to add some custom functionality to Excel, functionality that boils down to being custom functions and possibly a few custom actions that are event drive like Load Workbook.

I use to do custom vba work a few years back when Excel 2003 was the current version so I have experience with vba in Excel but under an older version.

My question is when you have 2 or more Excel users in a corporate environment who want access to the same custom Excel functions what is the best way to do this?

In Excel 2003 we had an Excel Add-in we distributed to each user and it worked great. However it looks like as if using Add-ins in 2007/2010 is much harder because of security lockdowns.

I also was wonderrng how portable is custom code in vba for Excel 2003 when going to 2007/2010?

Any thoughts or comments are most welcome.

Thanks