I am pregnant and have been really ill. I am an non-exempt salaried employee. Recently my boss came to me and told me that if I continue to miss days that I will be dropped to hourly. Is this possible?

Yes, it is possible. As a non-exempt employee you only have an expectation of being paid on an hourly basis.

The only way this would be illegal would be if you could provide valid evidence that a similarly situated employee who missed the same amount of time for a non-maternity condition would not be changed to hourly.

Please explain the difference between Exempt & Non-Exempt Salaried Employees.

Simplistically:

Exempt employees are guaranteed (with limited exceptions) a weekly salary of at least $455 per week without reductions related to quality or quantity of work, but are not eligible to receive overtime pay. Nonexempt employees need only be paid for the hours they work, but must receive overtime pay when they work in excess of 40 hours in a workweek.

All employees are deemed nonexempt to start. If the employee's job duties qualify them for exempt status, then the employer can, at their discretion, treat them as exempt.

See here for the four exempt classifications and the criteria for each:
http://www.dol.gov/esa/regs/compliance/whd/fairpay/fs17a_overview.htm

I am a salaried employee at a hotel. I do not get paid for overtime. It has never been made clear to me if I am exempt or non-exempt. I recently had to take three days off for medical reasons. My employer only paid me for the number of days that I worked and not my weekly salaried amount. Is this legal?

Again, "salaried" is merely a pay method. We don't have enough information to answer.

What exactly are your duties there? Does your employer have a sick pay benefit and, if so, how many days per year and did you have any sick pay hours left to draw on?

I am the Executive Housekeeper. We do not receive sick day pay.

You can check out the classifications of exempt employment here. I would imagine, if anything, you could be considered as exempt either under the Executive or Administrative criteria.
http://www.dol.gov/esa/regs/compliance/whd/fairpay/fs17a_overview.htm

Now, if you ARE exempt, since the employer does not provide a sick pay plan, your salary cannot be docked when you are absent due to illness or injury.
See paragraph (a)(1)(3) here:
http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_541/29CFR541.118.htm

If you do not qualify as an exempt employee, then the law does not require you be paid for anything other than actual hours worked.