Hi sjk,
There is actually a field in the Customer(lead) table labelled CreatedDate, this date is populated when you enter a new lead into Method and I believe this is what you are looking for. By default this field isn't available on the Edit/New Leads screen, however this field could be added to either of these screens if that is what you are looking for. Another alternative would be to go to Customize->IntegrationTools, then select the Import/Export Tool. From here you can choose the option to export the Customer Lead table, you can select whatever fields you want to export from here, one of these fields being CreatedDate.
In regards to seeing the date for Leads that you have converted to Customers, there is a field in the Customer table labelled LeadConvertedDate. As I suggested for the Leads, although this field isn't included on the New/Edit Customer screen you can Customize them to add this field. You can export this table the same way as explained above, except instead of choosing the CreatedDate field, make sure to include the LeadConvertedDate.
You also asked if these dates are available on Reports in Method, by default I don't believe this is currently available on our stock reports. However, I believe through customization, this field can be added to the them, like the Lead Report, found in the Leads tab. I hope this helps. I have provided a link here for some information on our Consulting services, if you find you want these fields added to your screens, or Reports, but would prefer to have someone set this up for you, you can get in touch with them and they should be able to help you out.
-Ben