Is it semantically correct to represent sub-sections using fieldsets?
= Basic Info =
First Name: ________
Last Name: ________
= Address =
Business Name: ________
Streeet: ________
City: ________
How do you deal with nested form sections? Isn't there any better way which would also look okay by default?
Yes, it's semantically correct.
http://dev.w3.org/html5/spec-preview/the-fieldset-element.html