I'll take a stab as this one...
Legally, you have to comply with FAR 91.103. The mechanisms that you choose are up to you; whether you get a phone briefing, a briefing from a licensed dispatcher, a weather packet from a certified vendor (i.e. Universal), or you derive your data from internet sources.
FltPlan.com is a legal conduit for this because its data is derived from official FAA sources and QICP certified. Aviationweather.gov (ADDS) is now QICP certified as well.