If you use the forum search feature, you'll find tons of similar threads. The short answer is we don't know for sure...end of June seems like a good guess, but it's still just a guess. If you want more details, use the forum search to find other threads about this (btw, for future reference, it's best to search for similar threads before creating a new one).
Apple almost never tells of updates ahead of time. It's only rumors an speculation until it happens. The Guide can be a good source of information for when an update might be released and is based off of the past trends of Apple. The only reason that they might wait until June specifically is to release something to coincide with WWDC.
My advice is that if you need it go out and buy it or wait if you feel more comfortable to see what June has to offer. Personally I helped my mom buy an iMac just before the switch from Core Duos to Core 2 Duos. Her order was delayed a week and a half for no reason and then suddenly her order was changed to that of a Core 2 Duo with better specs and slightly cheaper than it was going to be.
When the new Santa Rosa PCs SHIP you can start the clock on the new MacBooks coming soon.
Until then, announcements of future products are just MacBook fantasies to jack off to.
If the PC world cannot ship enough Santa Rosa notebooks, it doesn't bode well for new MacBooks -- if they ship and are everywhere, the Macs may be coming in 4-8 weeks.
Edit: Remember, Apple needs a huge supply of chipsets and parts to ship the machines with a guaranteed supply of hundreds of thousands to multiples of that in order to ship new MacBooks and iMacs. They really eat up major numbers of new parts compared to other PC manufacturer who needs enough for a production batch run.
All threads beginning with "When is, When you do you think, When will, When can we, At what moment, June, New, Should I, How long, October" or containing the abbreviation "WWDC" will now be ignored.