VBA will be back in Excel 2011 (not sure of version, whatever the Mac version of Excel 2010). For previous version of Mac, the VBA is about equivalent to Excel *97* - something like VB 5.0, I think. Not sure how up to date the VB in 2011 will be.
I use a Mac with Excel 2004. Its very similar to 2003, except that as starl mentioned, it uses VBA 5. Thus no Split, no Join, no vba Replace, no Dictionary and a few other things. You can use conditional compiling to provide any emulators that you might need.
In addition to the weak VBA, Mac does not support ActiveX controls.
On the other hand, AppleScript (via Macscript command) is a nice plus.
If you have specific issues, PM me. Or post it.
Thank you . it's very good to hear that VBA will be back in 2011.