If you're using Acrobat 5 (which it sounds like), then after using the Free Text tool to create your bit of text, click somewhere else then right-click on the text box, so you get the context menu. Select Properties. Select Border Color. Select White. This changes the text to white as well as the border. Then the next time you use the Free Text tool it will have the same properties (i.e. white text) - though you might find it difficult typing if you can't see what you're typing, so you might want to add all the text first, then go through changing the Border Color to white.
If you're using Acrobat 6, it's a similar process, but your best off (IMO) using the Properties Toolbar and you'll have to change the background colour, the border colour and the text colour to white.
Hope that helps. I checked and the text is searchable. I also checked with field text in Acrobat 5 and it doesn't seem to be included in a search, so you're best off using the free text tool, I think.