I would suggest that the convenience of annual payments is actually for the vendor - especially early stage firms who haven't yet automated charging clients. The only exception I can think of is if you are willing to give clients net 30 terms on the annual charge. I suspect you might see larger companies take this up.

In the common B2B world (often under $250/month) I suspect most buyers will see annual payment as a contract that handcuffs them and will slow down the buying cycle because of the larger cost. This could be as simple as additional approvals for a spend above $1000 or purely psychological.

For example I have seen Salesforce.com run into this with small offices, even after they do a successful trial they suffer from sticker shock when they see the annual per seat rate they are quoted. If Salesforce.com had automatically moved these same businesses into a monthly per seat charge on a credit card they wouldn't have batted an eye but instead it drags the buying cycle out.