Be careful about statements regarding on-the-job learning.

Employers don't like to be reminded of the time and expense required to train new employees.

Weak:

Although I have not worked with Lotus spreadsheets, I am a fast and willing learner.

Better:

As for your requirement for spreadsheet experience, I've worked with Excel, a program even more advanced than Lotus. Thus, I have no doubt I can quickly get up to speed on Lotus.