Update: The free-upgrade promotion for ReSharper 5 started on October 15th, so you are safe to commence buying licenses 
I contacted JetBrains today because I am looking to buy a ReSharper 4.5 license but I’m hesitant to buy it if the release of version 5 just around the corner. I emailed sales and this is the response I got:
Hello Jack,
Yes, there will be a promotion period 1-2 months before ReSharper 5 will be released. A certain release date is not known yet, but it is said to be in Q4/2009.
If you have any additional questions, I am happy to help.
Best regards.
If things do go to plan and JetBrains hit Q4, that means we will hopefully see the “promotion period” (i.e. free upgrade) start at the latest in November 2009. I am hopeful it will be before that because I am not sure I can last much longer without my ReSharper fix! 
One thing I remain curious about: Are JetBrains going to branch ReSharper into two versions: one each for pre/post VS2010? Because there is no way they are going to be able to back-port all the amazing features that are possible with WPF in VS2010. Makes me suspect that ReSharper 5 is still going to predominantly target VS2008 (with VS2010 compatibility), and ReSharper 6+ will be VS2010-only.
#1 by Stefan Breitenfeld on November 17, 2009 - 7:04 am
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The promotion period for ReSharper 5.0 already started on October 15th, so Q4 seems very likely now.
The VS2010 beta test is running for several months now, so be sure that all VS2010 features were communicated by Microsoft to their software partners long ago…
#2 by Jack on November 17, 2009 - 4:32 pm
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Stefan, yes you beat me to it
I will update the post
#3 by Stefan Breitenfeld on November 17, 2009 - 7:20 pm
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However, JetBrains haven’t announced a final release date so far, though they already have a “5.0 early access program”. As a web designer I’m waiting for the promised HTML support in v5.0, so if you happen to learn anything about a release date please be so kind to keep us informed.