No, it basically means two things

1: If you call Microsoft for support, they won't talk to you unless you have the minimum service pack installed with the possible exception of helping you get that service pack installed

and

2: Future updates will not be tested to ensure they work on systems without the specific service pack baseline. So all future security updates will likely require Vista SP1 and XP SP3, and will not install on systems "below" that level.

It is, however, a good idea to download the standalone service pack install files, and keep them somewhere safe. You might even want to look into creating a slipstreamed install disc. But at a minimum, look for the network professional version of the latest service pack for whatever OS you're running. You also want to make sure of any requirements. Service packs used to be cumulative, but starting with XP SP3, they started requiring previous service packs to be installed. SP3 requires SP1 be installed, so you'll need the SP1 or SP2 installer to get SP3 installed. I'm not sure the particulars of Vista SP2, but if it requires SP1, you'll want to download and archive SP1 and SP2.