I've had another thought.
Every employer has a legal duty to protect the health and welfare of it's employees under the health and safety at work act.
Punishing employees for not returning to work before they are well again sounds wreckless to me.
The Health and safety Executive web site will have guidance on the health and safety at work act along with this swine flu advice.
Swine flu - HSE News Announcement
The department of health also has advice for employers which recommends sending people home who display symptoms and not return to work until they are well.
Pandemic flu guidance for businesses: risk assessment in the occupational setting : Department of Health - Publications
For an employer not to follow the above advice would seem pretty poor in my opinion and would probably give good grounds for grievance procedures against your employer if they penalised you. And, if your employer is large enough then the HSE might want to have a word with them about not protecting the welfare of it's workers.
All of this is in my opinion of course so specific advice should be sought from the above departments Particularly HSE and DH.
HTH
-Neil