A validation rule's formula should be true when rules fails, so we need to have criteria for each failure condition, combined with an OR ( "||" ).  If I read your table correctly, here's how to express your requirements.  I used inline comments to highlight each LeadSource's section of the formula.

/* first check Wholesaler */
( ISPICKVAL(LeadSource, 'Wholesaler')
  &amp;&amp; ( ISBLANK(Lead_source_Details__c)
       || (ISPICKVAL(Lead_Source_Details__c, 'Other') &amp;&amp; ISBLANK(TextField__c))
     )
) ||
/* Personal Referral has no other requirements */
/* check Trade Show */
( ISPICKVAL(LeadSource, 'Trade Show/Event') &amp;&amp; ISBLANK(TextField__c)
) ||
/* check TBP */
( ISPICKVAL(LeadSource, 'TBP Event') &amp;&amp; ISBLANK(TextField__c)
) ||
/* Website has no other requirements */
/* check Other */
( ISPICKVAL(LeadSource, 'Other') &amp;&amp; ISBLANK(TextField__c)
)


Note that I didn't have the name of your text field, so I used TextField__c; update for your field name.

Three tier Validation Rule / Validation chain?
I'm looking for a low power clocking solution and it looks like crystals bottom out at 32 kHz.  Are there any low power (nA) solutions (crystals/ceramic resonators/custom transistor oscillator circuitry) that operate on nanoAmps of power in the single digit kHz range or lower?

EDIT

I wanted to summarize the facts from the answers below as well as my own research:


Crystals do not commercially exist lower than the standard 32 kHz flavor, due to size/resonance constraints of the quartz used internally (thanks to Olin Lathrop)
For a 32 kHz clock solution in the 100s of nA range, this oscillator IC could be used (thanks to stevenvh)
For lower frequencies (but not necessarily nA current consumption) many silicon oscillators, frequency synthesizers, PLLs, or real time clocks include internal clock divider circuitry and can be used to generate clocks as "slow" as 1 Hz.


So there is no solution which satisfies both of the constraints of sub-32 kHz &amp; sub-\$\mu\$A operation, but individual solutions that will satisfy one or the other.
